Document System.PlatformID.Other (dotnet#4897)

Co-authored-by: carlossanlop <[email protected]>

Author: carlossanlop

xml/System/PlatformID.xml

@@ -132,7 +132,7 @@
       </ReturnValue>
       <MemberValue>7</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Any other operating system. This includes Browser (WASM).</summary>
       </Docs>
     </Member>
     <Member MemberName="Unix">