Interop doc issues (dotnet#7436)

* Update Marshal.PtrToStructure doc.

Fixes dotnet#1024

* Document support is missing on non-Windows and Mono runtime.

Fixes dotnet#869

Co-authored-by: Elinor Fung <[email protected]>

Author: AaronRobinsonMSFT

xml/System.Runtime.InteropServices/DefaultDllImportSearchPathsAttribute.xml

@@ -66,7 +66,8 @@
 2.  Otherwise, if the attribute is applied to the assembly that contains the platform invoke, use the values specified by that instance of the attribute.  
   
 3.  Otherwise, search the assembly directory and then call the `LoadLibraryEx` function with the `LOAD_WITH_ALTERED_SEARCH_PATH` flag.  
-  
+
+  This attribute has no effect on non-Windows platforms or the Mono runtime.
  ]]></format>
     </remarks>
     <altmember cref="T:System.Runtime.InteropServices.DllImportSearchPath" />

xml/System.Runtime.InteropServices/Marshal.xml

@@ -7316,7 +7316,7 @@ On .NET 6 and later versions, this method is functionally equivalent to <xref:Sy
 
 ## Remarks  
  <xref:System.Runtime.InteropServices.Marshal.PtrToStructure%2A> is often necessary in COM interop and platform invoke when structure parameters are represented as an <xref:System.IntPtr?displayProperty=nameWithType> value. You cannot use this overload method with value types.  
-  
+ If the `ptr` parameter equals <xref:System.IntPtr.Zero?displayProperty=nameWithType>, `null` will be returned.
  ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentException">Structure layout is not sequential or explicit.  
@@ -7395,7 +7395,7 @@ On .NET 6 and later versions, this method is functionally equivalent to <xref:Sy
 
 ## Remarks  
  <xref:System.Runtime.InteropServices.Marshal.PtrToStructure%2A> is often necessary in COM interop and platform invoke when structure parameters are represented as an <xref:System.IntPtr?displayProperty=nameWithType> value. You can pass a value type to this overload method. In this case, the returned object is a boxed instance.  
-  
+ If the `ptr` parameter equals <xref:System.IntPtr.Zero?displayProperty=nameWithType>, `null` will be returned.
 
 
 ## Examples  
@@ -7490,7 +7490,7 @@ On .NET 6 and later versions, this method is functionally equivalent to <xref:Sy
 
 ## Remarks  
  <xref:System.Runtime.InteropServices.Marshal.PtrToStructure%60%601%28System.IntPtr%29> is often necessary in COM interop and platform invoke when structure parameters are represented as <xref:System.IntPtr?displayProperty=nameWithType> values. You can pass a value type to this method overload.  
-  
+ If the `ptr` parameter equals <xref:System.IntPtr.Zero?displayProperty=nameWithType>, `null` will be returned.
  ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentException">The layout of <typeparamref name="T" /> is not sequential or explicit.</exception>
@@ -7555,7 +7555,7 @@ On .NET 6 and later versions, this method is functionally equivalent to <xref:Sy
 
 ## Remarks  
  <xref:System.Runtime.InteropServices.Marshal.PtrToStructure%60%601%28System.IntPtr%2C%60%600%29> is often necessary in COM interop and platform invoke when structure parameters are represented as <xref:System.IntPtr> values. You cannot use this method overload with value types.  
-  
+ If the `ptr` parameter equals <xref:System.IntPtr.Zero?displayProperty=nameWithType>, `null` will be returned.
  ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentException">Structure layout is not sequential or explicit.</exception>