revise section on multi-statement support

The SQL that you Exec/Query/Etc is sent to the server whether it's one statement, two statements, a haiku, or the collected works of William Shakespeare.  I believe the driver *must* send it up in a single round-trip (ie, I don't think it can actually attempt to parse and split the query on the sly), but the backend can decide how to execute it.

For instance, I know that multi-statement execs work with PostgreSQL and lib/pq for N statements, as this is how `sqlx.LoadFile` (naively) works;  when I discovered this behavior was particular to PostgreSQL and not shared by SQLite and MySQL, that API had already been added and I didn't want to break it, even though it doesn't work reliably.

I've also expanded the transaction section to mention the fact that you can completely hose a connection if you don't execute statements serially, and removed a bit about releasing resources from a Result, which never has any associated connection resources (Exec returns them immediately).