Doc: libcrmcommon: PCMK_VALUE is not strictly for option values

Keeping them in options.h vs. xml_names.h is questionable since we often
use them for XML attributes that don't represent user-configurable
options. However, I have no problem reserving xml_names.h for element
and attributes names.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>