* Documentation/user/page.itely (Displaying spacing): use \book

* Documentation/user/page.itely (Displaying spacing): use \book
explicitly, so arrows show .
(Line breaking): add note about Forbid_line_break_engraver

* Documentation/user/tweaks.itely (Common tweaks): also set length
to prevent tie squeezing.
(Default files): use @var{}

* Documentation/user/converters.itely (Invoking abc2ly): remove mup2ly.
(Generating LilyPond files): add FOMUS.