Add warning about using \relative with tagged music (3253) (issue 7798045)

[dak]

https://codereview.appspot.com/7798045/diff/1/Documentation/notation/input.itely
File Documentation/notation/input.itely (right):

https://codereview.appspot.com/7798045/diff/1/Documentation/notation/input.itely#newcode2247
Documentation/notation/input.itely:2247: @end example
That's \relative without reference pitch...  This particular form would
not actually be used, instead music would be defined as
music =
\relative c' { ...

So I don't think these examples really make things more clear.  There
also is no "correct" and "incorrect" usage.

It is just that it rarely makes sense to apply \relative to music
treated with \keepWithTag/\removeWithTag since then different sequences
will be run through \relative.

However, if you just cut away material at the end of a sequence, or just
cut away articulations, this is not actually a problem.

So the rule more or less is
"Calling \relative on a sequence gained with \keepWithTag/\removeWithTag
will consider only the pitches actually remaining in the sequence,
possibly changing the octave relations."

https://codereview.appspot.com/7798045/