Added a note for the usage of abs() with ::MIN

[robertg]

There wasn't a note for the behaviour of ::MIN with abs, this PR should fix that.

• Should close Specify negative-returning behavior of abs method #16707

[lilyball]

This makes it sound like this behavior is restricted solely to int. It should in fact happen with any 2's-complement signed integral type (such as i8, i16, etc).

[robertg]

@kballard I was thinking about that, would "signed integer" cover all of those types?

The reason I used int was because of another doc note:

rust/src/libcore/num/mod.rs

Line 297 in d16a5cd

/// For int:

Should I also fix that usage of int?

[klutzy]

Hmm, my main interest in #16707 was that we need to decide general semantic of abs method rather than specific built-in types. I'll describe the issue more at #16707.
Anyway, the patch describes current situation which clearly improves documentation. Thanks!