doc: properly describe --enable-checking behavior

doc: properly describe --enable-checking behavior

This patch rewords the whole description to fix minor issues:
 - documents 'gimple' and 'types' checks,
 - clarifies what happens when option is used without '=list',
 - fixes inaccurate wrong wording about release snapshots,
 - describes that release checks can only de disabled explicitly.

gcc/ChangeLog:
	* doc/install.texi (--enable-checking): Properly document current
	behavior.
	(--enable-stage1-checking): Minor clarification about bootstrap.