Copy-editing of Debian documentation commit.

Some tweaks to the wording of the original pull request. Give up on following
the PGXS style of naming the README. This seems like something of marginal
benefit, and breaks Github's inline rendering of the README rst. I considered
just using a symlink instead, but decided against doing that.
Don't fingerpint PlannedStmt's relationOids or invalItems.

These fields related to dependencies that a plan has on relations, for the
purposes of plan cache invalidation. Apart from simply being superfluous,
jumbling these fields caused incorrect behavior; sometimes Oid references in
quals could appear here too, on the off chance that the Oid actually referred to
a relation. It wasn't apparent to the core system that the Oids didn't actually
reference relations, but for the purposes of cache invalidation that wouldn't
have mattered. This tended to result in many distinct hashtable entries for what
ought to have been considered the same plan by pg_stat_plans' fingerprinting
logic.

Per report from Michael Renner.
Bump PGSP_FILE_HEADER.

Last commit necessitated this.
Add "from_our_database" convenience column to pg_stat_plans view.

No need to bump PGSP_FILE_HEADER, since the on-disk serialization of the hash
table hasn't altered.
Fix typo.
Tweak pg_stat_plans_explain() semantics for the convenience of tools.

Previously, we warned in the event of a query text that produced a different
plan than that stored for the entry, while still returning the new plan, with
another warning inline, in the explain text. This warning is likely to hamper
third party parsing tools. Moreover, this behavior wasn't particularly
intuitive, since the user asked for the plan text for a particular entry/plan,
and not a particular query text. Just return NULL instead, while continuing to
raise a WARNING (or, if search_paths differ, ERROR) as before.
Fix typo.
Rename some columns in pg_stat_plans_queries convenience view.

Previously, the naming applied to certain columns suggested that they were in
some sense "histograms", which is not the case. I'll take the opportunity to
address this before a stable release of pg_stat_plans, which is likely to be
less than a week away.
calls_per_plan, not calls.
Update README.rst

Fix plan_ids column name in pg_stat_plans_queries docs
Merge pull request #8 from terrorobe/patch-2

Update README.rst
Tweak descriptions within pg_stat_plans_queries view's documentation.
Rename pg_stat_plans README to better match PGXS style guidelines.

It was previously less evident than it should have been that the README.rst file
belongs to pg_stat_plans. This could cause confusion because PGXS extension
documentation is all stored in the same directory (after installation, when
listed in the Makefile).

Per gripe from Cédric Villemain.
Revert "Rename pg_stat_plans README to better match PGXS style guidelines."

This reverts commit 062fe6780f34353951072d0afe2d7d524350424b.
Use MODULEDIR PGXS variable to avoid ambiguously named README.rst.

The problem fixed by this was that it was not immediately apparent which module
README.rst related to once installed into the $prefix/share/extension directory.
I couldn't just change the filename of README.rst because to do so would make
Github cease expanding the README as documentation on the project page.

By setting MODULEDIR, the README appears within $prefix/share/doc/pg_stat_plans,
thus preventing any possible collisions with other modules, and making it
immediately apparent what module the file belongs to.

Per gripe from Cédric Villemain.
Merge branch 'master' into debian
Fix MODULEDIR change