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.

doc/src/sgml/ref/explain.sgml

@@ -408,7 +408,7 @@ EXPLAIN ANALYZE EXECUTE query(100, 200);
    ->  Index Scan using test_pkey on test  (cost=0.29..9.29 rows=50 width=8) (actual time=0.039..0.091 rows=99 loops=1)
          Index Cond: ((id > $1) AND (id < $2))
  Planning time: 0.197 ms
- Total runtime: 0.225 ms
+ Execution time: 0.225 ms
 (5 rows)
 </programlisting>
   </para>