ux.texi: move "Quoting" and "Fix-it hints" from DiagnosticsGuidelines wiki page

ux.texi: move "Quoting" and "Fix-it hints" from DiagnosticsGuidelines wiki page

gcc/ChangeLog:
	* doc/ux.texi (Quoting): New subsection, adapted from material at
	https://gcc.gnu.org/wiki/DiagnosticsGuidelines written by
	MartinSebor and ManuelLopezIbanez.
	(Fix-it hints): Note that fix-it hints shouldn't be marked for
	translation.

Co-Authored-By: Manuel López-Ibáñez <manu@gcc.gnu.org>
Co-Authored-By: Martin Sebor <msebor@redhat.com>
From-SVN: r265606