Doc: NR clarified \footnote command as a TextScript (issue 6137050)

[graham]

http://codereview.appspot.com/6137050/diff/1/Documentation/notation/input.itely
File Documentation/notation/input.itely (right):

http://codereview.appspot.com/6137050/diff/1/Documentation/notation/input.itely#newcode1050
Documentation/notation/input.itely:1050: footnote is being attached but
can also be attached as a
isn't a textscript also a grob?  if so, you don't need to add that in
the text.  The example carries that story.

http://codereview.appspot.com/6137050/