* Documentation/user/changing-defaults.itely (Page layout):

* Documentation/user/changing-defaults.itely (Page layout):
document them.

* scm/page-layout.scm (ly:optimal-page-breaks): use new parameters
to get sane spaces for titles.

* ly/book-paper-defaults.ly: new parameters aftertitlespace,
beforetitlespace, betweentitlespace.

* scm/page-layout.scm (ly:optimal-page-breaks): add
betweensystempadding also to fixed distance for the spring.

This fixes: morgenlied.ly and input/regression/beam-concave.ly