Based on feedback from @luke-jr. (Thanks!)
* Use "may" to indicate that full peers don't need to track unconfirmed txes.
* Change the example given for why non-mining peers might want to track
unconfirmed txes.
* Note that txes also get dropped from mempool when memory needs
to be freed.
* Remove paragraph about storing non-relay/non-mine txes.
Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.
The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
