pgsql: List wait events in alphabetical order

List wait events in alphabetical order

This changes the documentation, and the related structures so as
everything is consistent.

Some wait events were not listed alphabetically since their
introduction, others have been added rather randomly.  Keeping all those
entries in order helps in maintenance, and helps the user looking at the
documentation.

Author: Michael Paquier, Kuntal Ghosh
Discussion: https://postgr.es/m/20181024002539.GI1658@paquier.xyz
Backpatch-through: 10, only for the documentation part to avoid an ABI
breakage.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/5ef037cf0b9e130120c5a788f1f1048f6242b076

Modified Files
--------------
doc/src/sgml/monitoring.sgml    | 24 ++++++++++++------------
src/backend/postmaster/pgstat.c | 18 +++++++++---------
src/include/pgstat.h            | 12 ++++++------
3 files changed, 27 insertions(+), 27 deletions(-)