Add missing documentation for DbProviderFactories APIs (#3401)

* Add missing documentation for DbProviderFactories APIs

Introduced in https://github.com/dotnet/corefx/issues/20903

* minor fixes

Author: roji

xml/System.Data.Common/DbProviderFactories.xml

@@ -251,8 +251,8 @@
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <summary>Returns the invariant provider names for all registered instances of <see cref="T:System.Data.Common.DbProviderFactory" />.</summary>
+        <returns>The invariant provider names for all registered instances of <see cref="T:System.Data.Common.DbProviderFactory" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -283,10 +283,10 @@
         <Parameter Name="factory" Type="System.Data.Common.DbProviderFactory" Index="1" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="providerInvariantName">To be added.</param>
-        <param name="factory">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="providerInvariantName">The invariant provider name under which to register the provider.</param>
+        <param name="factory">The instance of the provider factory to be registered.</param>
+        <summary>Registers an instance of <see cref="T:System.Data.Common.DbProviderFactory" /> under the specified invariant provider name.</summary>
+        <remarks>If a provider is already registered under <paramref name="providerInvariantName" />, that registration will be overwritten.</remarks>
       </Docs>
     </Member>
     <Member MemberName="RegisterFactory">
@@ -316,10 +316,10 @@
         <Parameter Name="factoryTypeAssemblyQualifiedName" Type="System.String" Index="1" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="providerInvariantName">To be added.</param>
-        <param name="factoryTypeAssemblyQualifiedName">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="providerInvariantName">The invariant provider name under which to register the provider.</param>
+        <param name="factoryTypeAssemblyQualifiedName">The assembly-qualified name for a <see cref="T:System.Data.Common.DbProviderFactory" />.</param>
+        <summary>Registers a <see cref="T:System.Data.Common.DbProviderFactory" /> with the given assembly-qualified name under the specified invariant provider name.</summary>
+        <remarks>If a provider is already registered under <paramref name="providerInvariantName" />, that registration will be overwritten.</remarks>
       </Docs>
     </Member>
     <Member MemberName="RegisterFactory">
@@ -349,10 +349,10 @@
         <Parameter Name="providerFactoryClass" Type="System.Type" Index="1" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="providerInvariantName">To be added.</param>
-        <param name="providerFactoryClass">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="providerInvariantName">The invariant provider name under which to register the provider.</param>
+        <param name="providerFactoryClass">The type representing a <see cref="T:System.Data.Common.DbProviderFactory" />.</param>
+        <summary>Registers a <see cref="T:System.Data.Common.DbProviderFactory" /> with the given type under the specified invariant provider name.</summary>
+        <remarks>If a provider is already registered under <paramref name="providerInvariantName" />, that registration will be overwritten.</remarks>
       </Docs>
     </Member>
     <Member MemberName="TryGetFactory">
@@ -382,10 +382,11 @@
         <Parameter Name="factory" Type="System.Data.Common.DbProviderFactory" RefType="out" Index="1" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="providerInvariantName">To be added.</param>
-        <param name="factory">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="providerInvariantName">The invariant provider name to look up.</param>
+        <param name="factory">When this method returns, contains the <see cref="T:System.Data.Common.DbProviderFactory" /> associated with the specified invariant provider name, if the name is found; otherwise, <see langword="null" />. This parameter is passed uninitialized.</param>
+        <summary>Attempts to get the <see cref="T:System.Data.Common.DbProviderFactory" /> registered under the specified invariant provider name.</summary>
+        <returns>
+          <see langword="true" /> if a provider is registered under the specified invariant provider name; otherwise, <see langword="false" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -415,9 +416,10 @@
         <Parameter Name="providerInvariantName" Type="System.String" Index="0" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="providerInvariantName">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="providerInvariantName">The invariant provider name of the registration to remove.</param>
+        <summary>Removes a <see cref="T:System.Data.Common.DbProviderFactory" /> registration for the given provider invariant name.</summary>
+        <returns>
+          <see langword="true" /> if the registration is successfully found and removed; otherwise, <see langword="false" />. This method returns <see langword="false" /> if a registration for <paramref name="providerInvariantName" /> is not found.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>