PR preprocessor/15185, PR preprocessor/20989:

	* doc/cppopts.texi <-MT>: Update description of algorithm for
	computing default target.
	<-M, -MD>: Reword "basename" text.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@120679 138bc75d-0d04-0410-961f-82ee72b054a4