Corrections To Memory Pool Text

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.