docs: clarify that refs/notes/ do not keep the attached objects alive

docs: clarify that refs/notes/ do not keep the attached objects alive

`git help gc` contains this snippet:

  "[...] it will keep [..] objects referenced by the index,
  remote-tracking branches, notes saved by git notes under refs/notes/"

I had interpreted that as saying that the objects that notes were
attached to are kept, but that is not the case. Let's clarify the
documentation by moving out the part about git notes to a separate
sentence.

Signed-off-by: Martin von Zweigbergk <martinvonz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>