Expand documentation in session.py

class Phase(int, Enum):

	"""

	Session phases indicate what messages to expect and are impacted by received messages

	Users should not generally need to use these values, however an understanding of the

	state-flow they represent is useful for understanding some error exception

	raised by `Session` methods.

	CONNECT:

	  This phase is the starting phase of a session, during which a HELO/EHLO message may be

	  awaited with `Session.helo()`.

	MAIL:

	  This phase is entered after HELO/EHLO, during which a MAIL message may be awaited with

	  `Session.envelope_from()`.  The `Session.extension()` method may also be used to get

	  the raw MAIL command with any extension arguments, or any other extension commands

	  that the MTA does not support (if the MTA supports passing these commands to

	  a filter).

	ENVELOPE:

	  This phase is entered after MAIL, during which any RCPT commands may be awaited with

	  `Session.envelope_recipients()`.  The `Session.extension()` method may also be used to

	  get the raw RCPT command with any extension arguments, or any other extension commands

	  that the MTA does not support (if the MTA supports passing these commands to

	  a filter).

	HEADERS:

	  This phase is entered after a DATA command, while message headers are processed.

	  Headers may be iterated as they arrive, or be collected for later through the

	  `Session.headers` object.

	BODY:

	  This phase is entered after a message's headers have been processed.  The raw message

	  body may be iterated over in chunks through the `Session.body` object.

	POST:

	  This phase is entered once a message's body has been completed (or skipped).  During

	  this phase the message editing methods of a `Session` object or the `Session.headers`

	  and `Session.body` objects may be used.

	"""

	CONNECT = 1

class Before(Position):

	"""

	Indicates a relative position preceding a subject `Header` in a header list

	See `HeadersAccessor.insert`.

	"""

	subject: Header

class After(Position):

	"""

	Indicates a relative position following a subject `Header` in a header list

	See `HeadersAccessor.insert`.

	"""

	subject: Header