doc: Fix a few minor errors spotted by testers

This commit addresses a few minor errors that were spotted while testing
the GCC manual with a few people, and while working on wider changes.

gcc/ChangeLog:

	* doc/extend.texi (Formatted Output Function Checking): New
	subsection for  grouping together printf et al.
	(Exception handling) Fix missing @ sign before copyright
	header, which lead to the copyright line leaking into
	'(gcc)Exception handling'.
	* doc/gcc.texi: Set document language to en_US.
	(@copying): Wrap front cover texts in quotations, move in manual
	description text.