intl: Improve comments.
* gettext-runtime/intl/Makefile.am: Add an overview of the code.
* gettext-runtime/intl/loadinfo.h (_nl_make_l10nflist): Improve comment.
* gettext-runtime/intl/dcigettext.c (guess_category_value): Likewise.
* gettext-runtime/intl/finddomain.c (_nl_find_domain): Likewise.
