- Another pass through the series of error messages

- Another pass through the series of error messages
emitted when relationship() is configured with
ambiguous arguments.   The "foreign_keys"
setting is no longer mentioned, as it is almost
never needed and it is preferable users set up
correct ForeignKey metadata, which is now the
recommendation.  If 'foreign_keys'
is used and is incorrect, the message suggests
the attribute is probably unnecessary.  Docs
for the attribute are beefed up.  This
because all confused relationship() users on the
ML appear to be attempting to use foreign_keys
due to the message, which only confuses them
further since Table metadata is much clearer.

- If the "secondary" table has no ForeignKey metadata
and no foreign_keys is set, even though the
user is passing screwed up information, it is assumed
that primary/secondaryjoin expressions should
consider only and all cols in "secondary" to be
foreign.  It's not possible with "secondary" for
the foreign keys to be elsewhere in any case.
A warning is now emitted instead of an error,
and the mapping succeeds. [ticket:1877]

- fixed incorrect "Alternate Collection Mappings" reference
in the docs, not sure if someone wants to reference
"Rows that Point to Themselves" function
- "Collection Mapping" is "Advanced Collection Mapping", this
section is troublesome since nobody really needs it but it
is public API