Unhide RDoc documentation from top level Gem module

[nobu]

What was the end-user or developer problem that led to this PR?

The Gem module is not listed in the “Class and Module Index” at all.

What is your fix for the problem, implemented in this PR?

Move :stopdoc: directive at the top level in lib/rubygems/compatibility.rb in the module block.
If this is at the top level, it stops the documentation of the entire module, but not only the part in this file.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[deivid-rodriguez]

We discussed something somewhat similar at #5299, any ideas to prevent this kind of issue from being reintroduced?

[nobu]

Although rdoc --verbose shows a module Gem (undocumented) line, this is buried in a lot of similar messages.
I guess rdoc may be possible to warn when already documented class/module is ignored.
Or, simple git grep '^# *:stopdoc:' -- '*.rb' may help in many cases.

[deivid-rodriguez]

I run rdoc --verbose and I get three "module Gem (undocumented)" messages with or without this PR.

[nobu]

I think this is a bug of rdoc, a :stopdoc: in a file should not affect the same class/module in other files.

[deivid-rodriguez]

@nobu I'm looking into this and I'm not seeing the missing Gem module. When I run rdoc and open the generated docs, the Gem module is listed under "Class and Module Index". 🤔

[nobu]

Snapshot of the part of https://docs.ruby-lang.org/en/master/:
The next to GC::Profiler is Gem::AvailableSet, there should be Gem between them.

[deivid-rodriguez]

My bad.

Locally I get a slightly different result where namespaces are collapsed by default. So Gem is listed because it wraps other classes. However, unlike other namespaces, it's not clickable so docs are not generated for it.

After this PR, it becomes clickable