pgsql: Fix inadequacies in recently added wait events

Fix inadequacies in recently added wait events

In commit 9915de6c1cb2, we introduced a new wait point for replication
slots and incorrectly labelled it as wait event PG_WAIT_LOCK. That's
wrong, so invent an appropriate new wait event instead, and document it
properly.

While at it, fix numerous other problems in the vicinity:
- two different walreceiver wait events were being mixed up in a single
wait event (which wasn't documented either); split it out so that they
can be distinguished, and document the new events properly.

- ParallelBitmapPopulate was documented but didn't exist.

- ParallelBitmapScan was not documented (I think this should be called
"ParallelBitmapScanInit" instead.)

- Logical replication wait events weren't documented

- various symbols had been added in dartboard order in various places.
Put them in alphabetical order instead, as was originally intended.

Discussion: https://postgr.es/m/20170808181131.mu4fjepuh5m75cyq@alvherre.pgsql

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/030273b7ea468ed4b3073dfd1f2ad88e3129df6a

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 32 +++++++++++++++++--
src/backend/postmaster/pgstat.c | 36 +++++++++++++---------
.../libpqwalreceiver/libpqwalreceiver.c | 4 +--
src/backend/replication/slot.c | 3 +-
src/include/pgstat.h | 16 +++++-----
5 files changed, 64 insertions(+), 27 deletions(-)