- fix pypostgresql typo, [ticket:2185]

- fix pypostgresql typo, [ticket:2185]
- rewrite "getting a session" docs to delineate
all sessionmaker() use cases distinctly
- rewrite "Managing Transactions" doc to spell out
each specific step completely, [ticket:2204]
- add to create_engine() and main "engine" doc
an unambiguous statement that the create_engine()
does not create a connection
- other link fixes
- rewrite "deleting" section so that "deleting from collections"
is separate, well-described, [ticket:2202]
- rephrase "autocommit mode" into its own section, move it down,
put some bold text that you shouldn't be using it.
- simplify the "subtransactions" section and make it local to
"autocommit" since it is utterly unneeded by anybody.
- oracle is not doing 2-phase anytime soon