Document encode(bytea, 'escape')'s behavior correctly.

I changed this in commit fd15dba543247eb1ce879d22632b9fdb4c230831, but
missed the fact that the SGML documentation of the function specified
exactly what it did.  Well, one of the two places where it's specified
documented that --- probably I looked at the other place and thought
nothing needed to be done.  Sync the two places where encode() and
decode() are described.