Get session hash pad input bytes after format seen

The use of some raw pad bytes as part of the input to the session hash
is a feature of internal format 0, and therefore those bytes must be
taken only after format indicator has been handled.  This follows up
to commit 6774b320 to fix that.

* onetime:
  (Design Overview): Update where the session hash input bytes live.
  (PadSession._digest_source_length): This is just a length, so don't
    try to document where the initialization bytes are located.
  (PadSession._session_hash): Update documentation here too.
  (PadSession.prepare_for_encryption): Don't initialize hash here, and...
  (PadSession.prepare_for_decryption): ...update related comment here.
  (PadSession.convert): Don't initialize hash here either.
  (PadSession._make_inner_header, PadSession._handle_inner_header):
    Initialize session hash right after the fuzz source length bytes.

* check.sh: Update accordingly.