Suggest to "Open the singleton class with class << self" when Style/ModuleFunction is set to forbidden

[RDeckard]

Context:

Style/ModuleFunction:
  EnforcedStyle: forbidden

When extend self or module_function is used in this context ⬆️, the cop message might suggest to "Open the singleton class with class << self instead.".
WDYT?

---

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[bbatsov]

We rarely use messages that are multi-sentence, so I think it would probably be better to improve the cop documentation instead. I see we have no suggestions for the forbidden style at all there and in general the documentation is pretty basic to begin with.

[RDeckard]

I think it would probably be better to improve the cop documentation instead.

Good idea 👍, I've reverted the changes and updated the documentation with this suggestion + trying to make it a little more accurate about the extend self + private exception.

[bbatsov]

Thanks!