Doc: Remove redundant info from documentation of -ansi.

Doc: Remove redundant info from documentation of -ansi.

The -ansi option has essentially been superseded by the more general
-std= option, and all the additional information about its effects is
already covered elsewhere in the manual.  I also cleaned up some
confusing text about alternate keywords that I noticed while
confirming this.

gcc/ChangeLog
	* doc/extend.texi (Alternate Keywords): Clean up text and remove
	discussion of "restrict", which is not a GNU extension at all.
	* doc/invoke.texi (C Dialect Options): Remove detailed discussion.