doc: standardize helptext of numfmt and slice into single options

doc: standardize helptext of numfmt and slice into single options

* src/numfmt.c (usage): Correct synopsis and make command description
clearer.  Start option descriptions with lowercase letter; use
semicolon instead of period where needed; indent continuation lines;
gettextize single options for ease of translation and maintenance;
sort options alphabetically.
* doc/coreutils.texi (numfmt invocation): Sort numfmt options
alphabetically.  Enforce double-blank-after-period style.

This addresses http://bugs.gnu.org/13681.
Improved-by: Bernhard Voelker