Doc: Clarify footnotes as articulations (2971) (issue 6845078)

[dak]

http://codereview.appspot.com/6845078/diff/1/Documentation/notation/input.itely
File Documentation/notation/input.itely (right):

http://codereview.appspot.com/6845078/diff/1/Documentation/notation/input.itely#newcode1250
Documentation/notation/input.itely:1250: address@hidden \footnote
address@hidden @var{offset} @var{footnote} @var{music}
It is not really a "further form", and if you consider it a "further
form", the @var{direction} would not be optional.

This is a syntactic wart slated to go away eventually (you can already
write
xxx=\footnote ... -4
and then use
  c\xxx
without further ado, but it does not yet work in situ.  This is the same
issue as with -\tweak.

http://codereview.appspot.com/6845078/diff/1/Documentation/notation/input.itely#newcode1333
Documentation/notation/input.itely:1333: both a grob-name and a
direction indicator will generate an error.}
This warning is extremely misleading since footnotes with a grob-name
are never attached to anything in the music but rather to a moment of
time.

@warning {If you use a grob specification, the footnote affects
everything at the current time step like @code{\once \override} would.
Just like an override, it has to occur between @emph{complete} rhythmic
events without a directional modifier of its own and will affect all
grobs produced by the following event, whether caused by the basic event
or one of its articulations.}

http://codereview.appspot.com/6845078/