Add Notation appendix for context mod identifiers. (issue4794057)

[n . puttock]

Reviewers: ,

Message:
Hello,

This patch adds auto-generated documentation for context modifications
(currently, only \RemoveEmptyStaves).

Any context modification identifier defined in ly/context-mods-init.ly
will get an entry in the notation appendices so long as it has a
\description docstring.

Cheers,
Neil

Description:
Add Notation appendix for context mod identifiers.

* Documentation/notation/notation-appendices.itely:

  add node for context modifications

* lily/context-mod.cc:

  add type predicate ly:context-mod?

* ly/context-mods-init.ly (RemoveEmptyStaves):

  add docstring via \description

* scm/document-context-mods.scm:

  new file for generating context-mod documentation

* scm/document-identifiers.scm (identifier<?):

  move to documentation-lib.scm

* scm/documentation-generate.scm:

  load document-context-mods.scm and output documentation to
  context-mod-identifiers.tely

* scm/documentation-lib.scm (identifier<?):

  moved from document-identifiers.scm so it can be used in
  document-context-mods.scm too

* scripts/auxiliar/ref_check.tely:

  exclude context-mod-identifiers.tely

* scripts/build/website-known-missing-files.txt:

  add context-mod-identifiers.tely

Please review this at http://codereview.appspot.com/4794057/

Affected files:
  M Documentation/notation/notation-appendices.itely
  M lily/context-mod.cc
  M ly/context-mods-init.ly
  A scm/document-context-mods.scm
  M scm/document-identifiers.scm
  M scm/documentation-generate.scm
  M scm/documentation-lib.scm
  M scripts/auxiliar/ref_check.tely
  M scripts/build/website-known-missing-files.txt