Note cases when pinned formulae get upgraded.

[MikeMcQuaid]

CC @alexrussell @ilovezfs for thoughts.

Fixes #3021.

[alexrussell]

@MikeMcQuaid this is good, and thanks for adding it.

Being as you asked for my thoughts, personally, I feel that this message still doesn't make it absolutely clear in some situations.

In the ruby command help and Manpage I'd change it to something like this:

Pin the specified , preventing them from being upgraded when issuing the brew upgrade <formulae> command. See also unpin.
Please note that pinning a formula does not prevent it from being upgraded if a package that relies on it is upgraded, as we do not allow formulae to be built against non-latest versions.

The note in the FAQ is alright in terms of getting the point across (though I still feel like it's not quite as clear as it could be, but cannot offer a better revision right now).

That said, if you don't agree with my revisions here, having something is better than nothing so I'm happy enough with what you already have.

[MikeMcQuaid]

Cheers for the review and feedback. Want to try and keep the manpage terse for now so 🚢ing as is but may consider fleshing it out if there's future confusion.