Describe special values in GUC descriptions more consistently.

Many GUCs accept special values like -1 or an empty string to
disable the feature, use a system default, etc.  While the
documentation consistently lists these special values, the GUC
descriptions do not.  Many such descriptions fail to mention the
special values, and those that do vary in phrasing and placement.
This commit aims to bring some consistency to this area by applying
the following rules:

* Special values should be listed at the end of the long
  description.
* Descriptions should use numerals (e.g., "0") instead of words
  (e.g., "zero").
* Special value mentions should be concise and direct (e.g., "0
  disables the timeout.", "An empty string means use the operating
  system setting.").
* Multiple special values should be listed in ascending order.

Of course, there are exceptions, such as
max_pred_locks_per_relation and search_path, whose special values
are too complex to include.  And there are cases like
listen_addresses, where the meaning of an empty string is arguably
too obvious to include.  In those cases, I've refrained from adding
special value information to the GUC description.

Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: "David G. Johnston" <david.g.johnston@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/Z6aIy4aywxUZHAo6%40nathan

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