doc: Improve punctuation and grammar in -fdiagnostics-format docs

doc: Improve punctuation and grammar in -fdiagnostics-format docs

The hyphen can be misunderstood to mean "emitted to -" i.e. stdout.
Refer to both forms by name, rather than using "the former" for one and
referring to the other by name.

gcc/ChangeLog:

	* doc/invoke.texi (Diagnostic Message Formatting Options):
	Replace hyphen with a new sentence. Replace "the former" with
	the actual value.