libstdc++: Tweak comment on generated #endif lines in bits/version.h

libstdc++: Tweak comment on generated #endif lines in bits/version.h

Make the #endif comment match the #if condition in the generated code.

Also adjust a comment in the Scheme code which describes the logic. The
__glibcxx_NAME macro is not defined _unconditionally_, as it still
depends on the conditions in cxxmin, extra_cond etca,. and the
__cpp_lib_NAME macro now depends on no_stdname too.

libstdc++-v3/ChangeLog:

	* include/bits/version.tpl: Fix comment on #endif. Tweak
	description of when macros are defined.
	* include/bits/version.h: Regenerate.

Reviewed-by: Arsen Arsenović <arsen@aarsen.me>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>