doc/fast-import: clarify notemodify command

doc/fast-import: clarify notemodify command

The "notemodify" fast-import command was introduced in commit a8dd2e7
(fast-import: Add support for importing commit notes, 2009-10-09)
The commit log has slightly different description than the added
documentation. The latter is somewhat confusing. "notemodify" is a
subcommand of "commit" command used to add a note for some commit.
Does this note annotate the commit produced by the "commit" command
or a commit given by it's committish parameter? Which notes tree
does it write notes to?

The exact meaning could be deduced with old description and some
notes machinery knowledge. But let's make it more obvious. This
command is used in a context like "commit refs/notes/test" to
add or rewrite an annotation for a committish parameter. So the
advised way to add notes in a fast-import stream is:
1) import some commits (optional)
2) prepare a "commit" to the notes tree:
2.1) choose notes ref, committer, log message, etc.
2.2) create annotations with "notemodify", where each can refer to
a commit being annotated via a branch name, import mark reference,
sha1 and other expressions specified in the Documentation.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>