The manuals in doc/emacs, doc/lispintro and doc/lispref have no
translations, so filtering the per-directory language list by DOCLANGS
left those directories with nothing to build when DOCLANGS does not
include 'default'. These three manuals must be built and installed
regardless of DOCLANGS: the Emacs and Emacs Lisp manuals are
referenced from doc strings, and the Introduction to Emacs Lisp
Programming is an important enough part of Emacs to be available
whatever the language preferences of the user who builds the manuals.
* Makefile.in (MANDATORY_DOC_DIRS): New variable.
(FILTER_DOCLANGS): Use it to always build the default language for
those directories.
(INFO_NONMISC, srcdir_doc_info_dir_inputs): Say why these are not
gated by DOCLANGS.
* doc/translations/README: Document that those three manuals are
always built in the default language.
* doc/translations/README: Document how to compile all
translations, or just for one language. Clarify where Texinfo
source code resides, and where compiled output goes.
