further simplify pool-sharing doc

further simplify pool-sharing doc

1. the event based approach doesn't require dispose() to be
   called at all, and the note that the "pool will hang" makes
   no sense.   I have no idea what that refers towards
2. the subsequent paragraph about connections and sessions
   is unintelligible.  old paragraphs like these are likely
   why people complain about the docs so much.  try to just
   say "don't do this", as that is easier than trying to
   explain to use connection.invalidate() etc.

Change-Id: Id840c65a2f71583ced4dc82fd8690e7da4c4b10e