GTY: Enhance 'string_length' option documentation

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 2a967f3d3a45294640e155381ef549e0b8090ad4 (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 7f2935c734c36f84ab62b20a04de465e19061333 (Subversion r9191).

This amends commit f3b957ea8b9dadfb1ed30f24f463529684b7a36a
"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.