Doc: Add how to change the notation fonts (issue 330040043 by address@hidden)

[tisimst . lilypond]

https://codereview.appspot.com/330040043/diff/1/Documentation/notation/input.itely
File Documentation/notation/input.itely (right):

https://codereview.appspot.com/330040043/diff/1/Documentation/notation/input.itely#newcode2845
Documentation/notation/input.itely:2845: @end example
I think it would be prudent to show all the inputs in one place, at
least their defaults, so users know what acceptable. At the moment they
are:
- #:music
- #:brace
- #:roman
- #:sans
- #:typewriter

I know the text font keywords show up in text.itely, but I think they
should all show up together somewhere.

It is also important for users to understand that you can only apply
(set-global-fonts ... ) once per \book block since it resets the `fonts`
variable each time you call the function. The following does work,
though:

\paper {
  #(set-global-fonts ... )
}
\book { ... }

\paper {
  #(set-global-fonts ... )
}
\book { ... }

where each call to set-global-fonts only affects the book(s) following
it unless re-called as shown.

https://codereview.appspot.com/330040043/diff/1/Documentation/notation/input.itely#newcode2866
Documentation/notation/input.itely:2866: you can use them in the same
way as Gonville.
This is true, but at the moment, the only allowed font names must have
the following suffixes: -11, -13, -14, -16, -18, -20, -23, -26, -brace.
This is easily remedied by modifying the internal function, but for now
at the user level, this is a significant limitation because it assumes
that the music font has ALL the above optical sizes and/or brace font.
Not sure what information here is most appropriate, but I think we would
should at least raise this flag so users realize that you can't use just
any music font out there that might work with other apps (like Sibelius,
Finale, Dorico, etc.).

https://codereview.appspot.com/330040043/