Issue 3926: Add \magnifyMusic; clarify fontSize in docs. (issue 96570043)

[markpolesky]

Reviewers: ,

Message:
I'd like to add this \magnifyMusic command, which gives users an easy
way to enter notes at different sizes, and scales stems and beams
automatically.  This is not an obscure need; this type of notation is
fairly common (at least in the mid- to late-Romantic piano repertoire),
and setting the fontSize context property alone is insufficient, since
stems and beams don't get scaled.

In the process of doing this, I ended clarifying the docs on fontSize.

https://code.google.com/p/lilypond/issues/detail?id=3926

Thanks.
- Mark

Description:
Add \magnifyMusic; clarify fontSize in docs.

Please review this at https://codereview.appspot.com/96570043/

Affected files (+293, -132 lines):
  M Documentation/notation/editorial.itely
  M Documentation/notation/spacing.itely
  M ly/music-functions-init.ly