GIT_TRACE_PACKET is not documented

While GIT_TRACE is documented on git manpage, and mentioned in RelNotes, GIT_MERGE_VERBOSITY is documented on git manpage and mentioned on git-merge manpage and other... GIT_TRACE_PACKET isn't documented _anywhere_:

  $ git grep GIT_TRACE_PACKET
  pkt-line.c:static const char trace_key[] = "GIT_TRACE_PACKET";