Update AssemblyLoadContext.xml (#2833)

- Fix AssemblyLoadContext.LoadUnmanagedDll return value. IntPtr is a value type and can't be null.
- Add missing property descriptions.

Author: MSDN-WhiteKnight

xml/System.Runtime.Loader/AssemblyLoadContext.xml

@@ -730,12 +730,12 @@ Each <xref:System.Runtime.Loader.AssemblyLoadContext> can load only:
       <Docs>
         <param name="unmanagedDllName">Name of the unmanaged library. Typically this is the filename without its path or extensions. </param>
         <summary>Allows derived class to load an unmanaged library by name.</summary>
-        <returns>A handle to the loaded library, or <see langword="null" />.</returns>
+        <returns>A handle to the loaded library, or <see cref="F:System.IntPtr.Zero" />.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
- The default implementation always return `null`. When `null` is returned, the runtime loads the library with its default policy.
+ The default implementation always returns <xref:System.IntPtr.Zero?displayProperty=nameWithType>. When <xref:System.IntPtr.Zero?displayProperty=nameWithType> is returned, the runtime loads the library with its default policy.
  
  This virtual method can be overridden to customize the unmanaged library search algorithm. When overridden the name can be used to identify the library. The loaded library does not need to match the requested name, but can be transformed as needed. This could include adjusting the name for platform specific naming.
  
@@ -893,9 +893,17 @@ This event is raised if the native library cannot be resolved by the default res
         <Parameter Name="directoryPath" Type="System.String" />
       </Parameters>
       <Docs>
-        <param name="directoryPath">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="directoryPath">The full path to the directory where the optimization profiles are stored.</param>
+        <summary>Sets the root path where the optimization profiles for this load context are stored.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[ 
+
+## Remarks		  
+
+The profile optimization improves the startup performance of the load context on multicore computers by performing background compilation of methods that are likely to be executed, based on profiles created during previous compilations. For more information, see <xref:System.Runtime.ProfileOptimization>.
+
+         ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="StartProfileOptimization">
@@ -924,9 +932,17 @@ This event is raised if the native library cannot be resolved by the default res
         <Parameter Name="profile" Type="System.String" />
       </Parameters>
       <Docs>
-        <param name="profile">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="profile">The name of the optimization profile.</param>
+        <summary>Starts the profile optimization for the specified profile.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[ 
+
+## Remarks		  
+
+The profile optimization improves the startup performance of the load context on multicore computers by performing background compilation of methods that are likely to be executed, based on profiles created during previous compilations. For more information, see <xref:System.Runtime.ProfileOptimization>.
+
+         ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="ToString">
@@ -946,8 +962,8 @@ This event is raised if the native library cannot be resolved by the default res
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <summary>Returns the string representation of this load context.</summary>
+        <returns>The string representation of this load context.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -1011,4 +1027,4 @@ This event is raised if the native library cannot be resolved by the default res
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>