restore legacy begin_nested()->root transaction behavior

Restored a legacy transactional behavior that was inadvertently removed
from the :class:`_engine.Connection` as it was never tested as a known use
case in previous versions, where calling upon the
:meth:`_engine.Connection.begin_nested` method, when no transaction were
present, would not create a SAVEPOINT at all, and would instead only start
the outermost transaction alone, and return that :class:`.RootTransaction`
object, acting like the outermost transaction.   Committing the transaction
object returned by :meth:`_engine.Connection.begin_nested` would therefore
emit a real COMMIT on the database connection.

This behavior is not at all what the 2.0 style connection will do - in 2.0
style, calling :meth:`_future.Connection.begin_nested` will "autobegin" the
outer transaction, and then as instructed emit a SAVEPOINT, returning the
:class:`.NestedTransaction` object. The outer transaction is committed by
calling upon :meth:`_future.Connection.commit`, as is "commit-as-you-go"
style usage.

In non-"future" mode, while the old behavior is restored, it also
emits a 2.0 deprecation warning as this is a legacy behavior.

Additionally clarifies and reformats various engine-related
documentation, in particular future connection.begin() which
was a tire fire.

Fixes: #6408
Change-Id: I4b81cc6b481b5493eef4c91bebc03210e2206d39

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