pgsql: Rename EXPLAIN ANALYZE's "total runtime" output to "execution ti

Rename EXPLAIN ANALYZE's "total runtime" output to "execution time".

Now that EXPLAIN also outputs a "planning time" measurement, the use of
"total" here seems rather confusing: it sounds like it might include the
planning time which of course it doesn't. Majority opinion was that
"execution time" is a better label, so we'll call it that.

This should be noted as a backwards incompatibility for tools that examine
EXPLAIN ANALYZE output.

In passing, I failed to resist the temptation to do a little editing on the
materialized-view example affected by this change.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5f86cbd714c9d43c0fbb43a7b172f77ebf429548

Modified Files
--------------
doc/src/sgml/perform.sgml | 20 +++++++-------
doc/src/sgml/ref/explain.sgml | 2 +-
doc/src/sgml/rules.sgml | 58 +++++++++++++++++++++++-----------------
src/backend/commands/explain.c | 6 ++---
4 files changed, 48 insertions(+), 38 deletions(-)