* lily/vaticana-ligature-engraver.cc (need_extra_space): renamed

	* lily/vaticana-ligature-engraver.cc (need_extra_space): renamed
	to need_extra_horizontal_space; bugfix: use class scope to avoid
	global namespace pollution

	* Documentation/user/instrument-notation.itely (ligatures): added
	comment on possible future syntax change and how to work around

	* lily/vaticana-ligature-engraver.cc, mf/parmesan-heads.mf
	(epiphonus, cephalicus): fine-tune shape of plica head for narrow
	intervals (fixes agnus dei example)

	* lily/include/mensural-ligature.hh: make comments clearer

	* lily/mensural-ligature.cc: line thickness of flexa shape
	(originally by Pal Benko; slightly modified)

	* Documentation/user/instrument-notation.itely (white mensural
	ligatures): selected a more illustrative example