pgsql: Describe special values in more GUC descriptions.

Describe special values in more GUC descriptions.

Commit 977d865c36 established some rules for mentioning special
values (e.g., -1, 0, an empty string) in GUC descriptions. A few
parameters either neglect to mention their special values or
describe them in a way that doesn't follow those rules. Most of
these parameters were added after the aforementioned commit. This
commit fixes them.

Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/aqMkIpRr62cVqtCZ%40nathan
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bd1244343332419df6b34be38883ee0af3227030

Modified Files
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src/backend/utils/misc/guc_parameters.dat | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)