Fix some obsolete information in src/backend/optimizer/README.

Constant quals aren't handled the same way they used to be.  Also,
add mention of a couple more major steps in grouping_planner.
Per complaint a couple months back from Etsuro Fujita.
Lock relation used to generate fresh data for RMV.

The relation should not be accessible to any other process, but it
should be locked for consistency.  Since this is not known to
cause any bug, it will not be back-patch, at least for now.

Per report from Andres Freund
Revert commit 0725065b37b8b0e9074a624a8d3e3ac1844fc820.

The previous commit was intended to make psql show the full path name when
doing a \s (history save), but it was very badly implemented and would show
confusing if not outright wrong information in many situations; for
instance if the path name given to \s is absolute, or if \cd commands
involving relative paths have been issued.  Consensus seems to be that
we don't especially need this functionality in \s, and certainly not in \s
alone.  So revert rather than trying to fix it up.  Per gripe from
Ian Barwick.

Although the bogus behavior exists in all supported versions, I'm not
back-patching, because the work created for translators (by change of
a translatable message) would probably outweigh the value of what is
after all a mostly-cosmetic change.