Docs: IR: fix documentation of 'style values

Docs: IR: fix documentation of 'style values

* Document the engraver that creates KeyCancellation

* Add missing 'style values for time-signature
  and note-head interfaces

* The line-spanner interface does not contain 'style

* Key interface no longer uses 'style

* Note-name interface does not use 'style

* Key-signature interface no longer uses 'style;
  'glyph-name-alist is used instead.

Signed-off-by: Patrick McCarty <pnorcks@gmail.com>