Improve Texinfo x-refs checking script

* move Lily-specific manuals names and cross-references macro names to
manuals_definitions.py, and add Internals reference and Snippets;

* add include path command-line option for less kludged files searching;

* warn user about automatically generated documentation;

* add convenient shorthands as Documentation/GNUmakefile targets:

   make check-xrefs  # check documentation in English
   make fix-xrefs # interactively fix doco in English

   make ISOLANG=yourLanguageCode check-xrefs # idem for translations
   make ISOLANG=yourLanguageCode fix-xrefs