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.
Improve the error message given for modifying a window with frame clause.

For rather inscrutable reasons, SQL:2008 disallows copying-and-modifying a
window definition that has any explicit framing clause.  The error message
we gave for this only made sense if the referencing window definition
itself contains an explicit framing clause, which it might well not.
Moreover, in the context of an OVER clause it's not exactly obvious that
"OVER (windowname)" implies copy-and-modify while "OVER windowname" does
not.  This has led to multiple complaints, eg bug #5199 from Iliya
Krapchatov.  Change to a hopefully more intelligible error message, and
in the case where we have just "OVER (windowname)", add a HINT suggesting
that omitting the parentheses will fix it.  Also improve the related
documentation.  Back-patch to all supported branches.
Misc GIN refactoring.

Merge the isEnoughSpace and placeToPage functions in the b-tree interface
into one function that tries to put a tuple on page, and returns false if
it doesn't fit.

Move createPostingTree function to gindatapage.c, and change its contract
so that it can be passed more items than fit on the root page. It's in a
better position than the callers to know how many items fit.

Move ginMergeItemPointers out of gindatapage.c, into a separate file.

These changes make no difference now, but reduce the footprint of Alexander
Korotkov's upcoming patch to pack item pointers more tightly.
Fix missing argument and function prototypes.

Not sure how I missed these in previous commit.
Merge branch 'master' into hstore