Clarify that lookup/resolve/descriptor are singleton class methods

[jez]

The Ruby documentation uses # exclusively to mean "an instance method," for example in the Array documentation, making its usage here in the protobuf docs misleading: using # here makes it seem like you're supposed to call this method on instances of the enum module, but there are none.

In general, writing it like this in terms of Foo::SomeEnum brings the example more in line with the rest of the examples which all operate on a hypothetical enum module.

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[Logofile]

Thanks for the submission. I'm running it past our Ruby person to confirm, and will roll it into the docs once I get that.

[Logofile]

Thanks for the PR! I incorporated the changes upstream (this repo is currently output-only), and they published this morning.