Fix a doc details about loading Native DLLs

- Note that `NativeLibrary.Load()` etc. must return the actual OS handle
- Note that `AssemblyLoadContext.LoadUnmanagedDllFromPath()` also returns the OS handle
  (also fix its exceptions).

Author: swaroop-sridhar

xml/System.Runtime.InteropServices/NativeLibrary.xml

@@ -38,7 +38,7 @@
       </Parameters>
       <Docs>
         <param name="handle">The native library handle to be freed.</param>
-        <summary>Frees a given a loaded library handle, or takes no action if the input handle is <see cref="F:System.IntPtr.Zero" />.</summary>
+        <summary>Frees a loaded library given its OS handle, or takes no action if the input handle is <see cref="F:System.IntPtr.Zero" />.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -62,7 +62,7 @@
         <Parameter Name="name" Type="System.String" />
       </Parameters>
       <Docs>
-        <param name="handle">The native library handle.</param>
+        <param name="handle">The native library OS handle.</param>
         <param name="name">The name of the exported symbol.</param>
         <summary>Gets the address of an exported symbol.</summary>
         <returns>The address of the symbol.</returns>
@@ -99,7 +99,7 @@ This is a simple wrapper around OS calls and does not perform any name mangling.
       <Docs>
         <param name="libraryPath">The name of the native library to be loaded.</param>
         <summary>Provides a simple API for loading a native library that wraps the OS loader and uses default flags.</summary>
-        <returns>The handle for the loaded native library.</returns>
+        <returns>The OS handle for the loaded native library.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="libraryPath" /> is <see langword="null" />.</exception>
@@ -131,7 +131,7 @@ This is a simple wrapper around OS calls and does not perform any name mangling.
         <param name="assembly">The assembly loading the native library.</param>
         <param name="searchPath">The search path.</param>
         <summary>Provides a high-level API for loading a native library.</summary>
-        <returns>The handle for the loaded library.</returns>
+        <returns>The OS handle for the loaded library.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
 
@@ -208,7 +208,7 @@ Only one resolver can be registered per assembly. Trying to register a second re
         <Parameter Name="address" Type="System.IntPtr" RefType="out" />
       </Parameters>
       <Docs>
-        <param name="handle">The  native library handle.</param>
+        <param name="handle">The native library OS handle.</param>
         <param name="name">The name of the exported symbol.</param>
         <param name="address">When the method returns, contains the symbol address, if it exists.</param>
         <summary>Gets the address of an exported symbol and returns a value that indicates whether the method call succeeded.</summary>
@@ -240,7 +240,7 @@ Only one resolver can be registered per assembly. Trying to register a second re
       </Parameters>
       <Docs>
         <param name="libraryPath">The name of the native library to be loaded.</param>
-        <param name="handle">When the method returns, the handle of the loaded native library.</param>
+        <param name="handle">When the method returns, the OS handle of the loaded native library.</param>
         <summary>Provides a simple API for loading a native library and returns a value that indicates whether the operation succeeded.</summary>
         <returns>
           <see langword="true" /> if the native library was loaded successful; otherwise, <see langword="false" />.</returns>
@@ -273,7 +273,7 @@ Only one resolver can be registered per assembly. Trying to register a second re
         <param name="libraryName">The name of the native library to be loaded.</param>
         <param name="assembly">The assembly loading the native library.</param>
         <param name="searchPath">The search path.</param>
-        <param name="handle">When the method returns, the handle of the loaded native library.</param>
+        <param name="handle">When the method returns, the OS handle of the loaded native library.</param>
         <summary>Provides a high-level API that loads a native library and returns a value that indicates whether the operation succeeded.</summary>
         <returns>
           <see langword="true" /> if the load operation was successful; otherwise, <see langword="false" />.</returns>

xml/System.Runtime.Loader/AssemblyLoadContext.xml

@@ -774,12 +774,19 @@ Each <xref:System.Runtime.Loader.AssemblyLoadContext> can load only:
       <Docs>
         <param name="unmanagedDllPath">The path to the unmanaged library.</param>
         <summary>Loads an unmanaged library from the specified path.</summary>
-        <returns>A handle to the loaded library.</returns>
-        <remarks>To be added.</remarks>
+        <returns>The OS handle for the loaded native library.</returns>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+
+The OS handle returned by this method can be used with methods of <xref:System.Runtime.InteropServices.NativeLibrary?displayProperty=nameWithType> class.
+
+         ]]></format>
+        </remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="unmanagedDllPath" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentException">The <paramref name="unmanagedDllPath" /> argument is not an absolute path.</exception>
-        <exception cref="T:System.IO.FileNotFoundException">The unmanaged library cannot be found.</exception>
+        <exception cref="T:System.DllNotFoundException">The library can't be found.</exception>
+        <exception cref="T:System.BadImageFormatException">The library is not valid.</exception>
       </Docs>
     </Member>
     <Member MemberName="Name">