Update lmdb-semantics.md

The normal lifecycle of a txn is txn_begin / txn_commit|txn_abort.

For read-only txns, reset/renew is an optimization when you are going to issue multiple read txns within a single thread. Otherwise, just abort it. (Your description sounds like you must reset before abort. You should just abort directly and forget about reset/renew unless
you actually have a use for it.)