GTY: Enhance 'string_length' option documentation

We're (currently) not aware of any actual use of 'ht_identifier's with NUL
characters embedded; its 'len' field appears to exist for optimization
purposes, since "forever".  Before 'struct ht_identifier' was added in
commit 2a967f3d (Subversion r42334), we had in
'gcc/cpplib.h:struct cpp_hashnode': 'unsigned short len', or earlier 'length',
earlier in 'gcc/cpphash.h:struct hashnode': 'unsigned short length', earlier
'size_t length' with comment: "length of token, for quick comparison", earlier
'int length', ever since the 'gcc/cpp*' files were added in
commit 7f2935c7 (Subversion r9191).

This amends commit f3b957ea
"pch: Fix streaming of strings with embedded null bytes".

	gcc/
	* doc/gty.texi (GTY Options) <string_length>: Enhance.
	libcpp/
	* include/symtab.h (struct ht_identifier): Document different
	rationale.

gcc/doc/gty.texi

@@ -217,6 +217,17 @@ struct GTY(()) non_terminated_string @{
 @};
 @end smallexample
 
+Similarly, this is useful for (regular NUL-terminated) strings with
+NUL characters embedded (that the default @code{strlen} use would run
+afoul of):
+
+@smallexample
+struct GTY(()) multi_string @{
+  const char * GTY((string_length ("%h.len + 1"))) str;
+  size_t len;
+@};
+@end smallexample
+
 The @code{string_length} option currently is not supported for (fields
 in) global variables.
 @c <https://inbox.sourceware.org/87bkgqvlst.fsf@euler.schwinge.homeip.net>

libcpp/include/symtab.h

@@ -29,9 +29,7 @@ along with this program; see the file COPYING3.  If not see
 typedef struct ht_identifier ht_identifier;
 typedef struct ht_identifier *ht_identifier_ptr;
 struct GTY(()) ht_identifier {
-  /* This GTY markup arranges that the null-terminated identifier would still
-     stream to PCH correctly, if a null byte were to make its way into an
-     identifier somehow.  */
+  /* We know the 'len'gth of the 'str'ing; use it in the GTY markup.  */
   const unsigned char * GTY((string_length ("1 + %h.len"))) str;
   unsigned int len;
   unsigned int hash_value;