Build documentation from top-level Makefile

...because recursive Make is not-so-nice, and the rules for the
documentation are few and brief.
Remove declaration of unused variable
read-rl: Adapt to newer readline versions

Recent readline versions try to use as much "const char*" parameters
as possible, so callbacks have to be redefined coherently. Also,
completion_matches has been renamed to rl_completion_matches, and
the old name deprecated.