[MLIR] Improve doc for -mlir-print-local-scope and unhide

This is a pretty important debugging option to stay hidden. Also,
improve its cmd-line description; the current description gives no hint
that this is the one to use to have locations printed inline.
Out-of-line locations are also unproductive to work with in many cases
where the locations are actually compact, which is also why this option
should be more visible.  This revision doesn't change the default on it
though.

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D117186