Expand comment on struct switchstr.

This patch, extracted from a larger change on gomp-4_0-branch, expands
a comment documenting struct switchstr in gcc.c.

2015-08-25  Thomas Schwinge  <thomas@codesourcery.com>
	    Joseph Myers  <joseph@codesourcery.com>

	* gcc.c (struct switchstr): Expand comment.

Co-Authored-By: Joseph Myers <joseph@codesourcery.com>

From-SVN: r227173

gcc/ChangeLog

+2015-08-25  Thomas Schwinge  <thomas@codesourcery.com>
+	    Joseph Myers  <joseph@codesourcery.com>
+
+	* gcc.c (struct switchstr): Expand comment.
+
 2015-08-25  Nathan Sidwell  <nathan@acm.org>
 
 	* config/nvptx/nvptx.c (nvptx_write_function_decl): Reformat.

gcc/gcc.c

@@ -3068,10 +3068,15 @@ execute (void)
    SWITCH_LIVE to indicate this switch is true in a conditional spec.
    SWITCH_FALSE to indicate this switch is overridden by a later switch.
    SWITCH_IGNORE to indicate this switch should be ignored (used in %<S).
-   SWITCH_IGNORE_PERMANENTLY to indicate this switch should be ignored
+   SWITCH_IGNORE_PERMANENTLY to indicate this switch should be ignored.
+   SWITCH_KEEP_FOR_GCC to indicate that this switch, otherwise ignored,
+   should be included in COLLECT_GCC_OPTIONS.
    in all do_spec calls afterwards.  Used for %<S from self specs.
-   The `validated' field is nonzero if any spec has looked at this switch;
-   if it remains zero at the end of the run, it must be meaningless.  */
+   The `known' field describes whether this is an internal switch.
+   The `validated' field describes whether any spec has looked at this switch;
+   if it remains false at the end of the run, the switch must be meaningless.
+   The `ordering' field is used to temporarily mark switches that have to be
+   kept in a specific order.  */
 
 #define SWITCH_LIVE    			(1 << 0)
 #define SWITCH_FALSE   			(1 << 1)