Adjusted proposal for grammar-like code block on extension page (#5857)

Follow-up proposal to #5856:

_Copying my thoughts from
#5856 (comment)

> @MaryaBelanger Can we keep the original symbols as well as the
comments? Otherwise to those that are familiar with the syntax or skim
past the comments, it might look like only one member definition is
allowed.
> 
> With both the syntax and the comments, all readers are accounted for,
plus the readers may become more familiar with the syntax as a result :)

Author: parlough

src/content/language/extension-methods.md

@@ -197,8 +197,8 @@ to avoid a name conflict when invoking an extension explicitly.
 Use the following syntax to create an extension:
 
 ```plaintext
-extension <extension name> on <type> { // extension-name is optional
-  (<member definition>) // can provide one or more member-definitions
+extension <extension name>? on <type> { // <extension-name> is optional
+  (<member definition>)* // Can provide one or more <member definition>.
 }
 ```