fill in todo's for GC APIs (#4728)

Author: Maoni0

xml/System/GC.xml

@@ -206,7 +206,7 @@
         <param name="length">Specifies the length of the array.</param>
         <param name="pinned">Specifies whether the allocated array must be pinned.</param>
         <summary>Allocates an array.</summary>
-        <returns>To be added.</returns>
+        <returns>An array object.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
@@ -250,7 +250,7 @@ If pinned is set to `true`, `T` must not be a reference type or a type that cont
         <param name="length">Specifies the length of the array.</param>
         <param name="pinned">Specifies whether the allocated array must be pinned.</param>
         <summary>Allocates an array while skipping zero-initialization, if possible.</summary>
-        <returns>To be added.</returns>
+        <returns>An array object with uninitialized memory except if it contains references or if it's too small for unpinned.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[