#2896 Document the new \undo function

[Anonymous]

Originally created by: *anonymous

Originally created by: tdanielsmusic

Document the new \undo function provided in committish
[r71eebe83138ead38e2e6383f6a9f145985e30b08]
under Issue 2874:

Provide an \undo function for turning overrides and sets into
reverts and resets
   
    \undo\hideNotes is equivalent to \unHideNotes, \undo\easyHeadsOn to
    \easyHeadsOff.  While this can't undo reverts, it is useful for a lot
    of overriding commands defined in ly/property-init.ly, and is also
    useful in combination with \hide/\omit.

[Google Importer]

Originally posted by: tdanielsmusic

Marking this as blocked on Issue 2870, as that is reworking the
sections into which this would be placed.

Blockedon: lilypond:2870