Allow #+, #-, #' to be cross referenced

[TheSmartnik]

Hi 👋

This small pr allows for methods like #+, #-, #* to be cross referenced.

This, for example, will improve ruby documentation. See Array#concat documentation

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I'd be happy to added tests with a little guidance on where this regex is currently tested

[TheSmartnik]

@aycabta tests seem to randomly fail on random versions.
Don't really think that this code is anything to do with this, but would love to hear your opinion

[aycabta]

@TheSmartnik Thank you for your reporting. The random failure of CI is fixed by #631.

I'd be happy to added tests with a little guidance on where this regex is currently tested

First, please add example code for it to test/xref_data.rb and test/xref_test_case.rb. Maybe, you should add C1#+ and @c1_plus. Finally, please add assesion(s) to TestRDocCrossReference#test_resolve_method of test/test_rdoc_cross_reference.rb.

[TheSmartnik]

@aycabta thanks for help! Added tests and rebased on master. All green now

[aycabta]

@TheSmartnik OK, thank you. I think that this feature is very important for documentation. I'm glad you propose this.