c.opt (Wmissing-attributes): Clean up doc string.

c.opt (Wmissing-attributes): Clean up doc string.

2019-02-25  Sandra Loosemore  <sandra@codesourcery.com>
	    Martin Sebor  <msebor@gmail.com>

	gcc/
	* c-family/c.opt (Wmissing-attributes): Clean up doc string.
	* common.opt (Wattribute-alias): Likewise.
	* doc/invoke.texi (Option Summary): List general form of
	-Wattribute-alias=.  List positive form of -Wmissing-attributes.
	(-Wmissing-attributes): Invert entry, rewrite and correct default.
	Add cross-references.
	(-Wattribute-alias): Rewrite and correct default.  Mention
	considered attributes (same as for -Wmissing-attributes).

Co-Authored-By: Martin Sebor <msebor@gmail.com>
From-SVN: r269196