Add docstrings to patroni.config (#2708)

Besides adding docstrings to `patroni.config`, a few side changes
have been applied:

* Reference `config_file` property instead of internal attribute
`_config_file` in method `_load_config_file`;
* Have `_AUTH_ALLOWED_PARAMETERS[:2]` as default value of `params`
argument in method `_get_auth` instead of using
`params or _AUTH_ALLOWED_PARAMETERS[:2]` in the body;
* Use `len(PATRONI_ENV_PREFIX)` instead of a hard-coded `8` when
removing the prefix from environment variable names;
* Fix documentation of `wal_log_hints` setting. The previous docs
mentioned it was a dynamic setting that could be changed. However
it is managed by Patroni, which forces `on` value.

References: PAT-123.

docs/patroni_configuration.rst

@@ -44,7 +44,6 @@ Some of the PostgreSQL parameters **must hold the same values on the primary and
 - **max_worker_processes**: 8
 - **max_prepared_transactions**: 0
 - **wal_level**: hot_standby
-- **wal_log_hints**: on
 - **track_commit_timestamp**: off
 
 For the parameters below, PostgreSQL does not require equal values among the primary and all the replicas. However, considering the possibility of a replica to become the primary at any time, it doesn't really make sense to set them differently; therefore, **Patroni restricts setting their values to the** :ref:`dynamic configuration <dynamic_configuration>`.
@@ -62,6 +61,7 @@ There are some other Postgres parameters controlled by Patroni:
 - **port** - is set either from ``postgresql.listen`` or from ``PATRONI_POSTGRESQL_LISTEN`` environment variable
 - **cluster_name** - is set either from ``scope`` or from ``PATRONI_SCOPE`` environment variable
 - **hot_standby: on**
+- **wal_log_hints: on** - for Postgres 9.4 and newer.
 
 To be on the safe side parameters from the above lists are not written into ``postgresql.conf``, but passed as a list of arguments to the ``pg_ctl start`` which gives them the highest precedence, even above `ALTER SYSTEM <https://www.postgresql.org/docs/current/static/sql-altersystem.html>`__