Log Message:
chat/matrix-synapse: Add comments related to workaround

  - add upstream bugtracker URL for patch to avoid use of Twisted private API

  - expand comment about the build system bug of installing fooorig,
    and leave a note that it should be changed to happen later.
    However, with the target as pre-install, the orig files are
    mysteriously still installed.

NFCI; this is a comment-only change (plus distinfo for the new comments)