pgsql: Revert use singular for -1 (commits 9ee7d533da and 5da9868ed9

Revert use singular for -1 (commits 9ee7d533da and 5da9868ed9

Turns out you can specify negative values using plurals:

        https://english.stackexchange.com/questions/9735/is-1-followed-by-a-singular-or-plural-noun

so the previous code was correct enough, and consistent with other usage
in our code.  Also add comment in the two places where this could be
confused.

Reported-by: Noah Misch

Diagnosed-by: 20210425115726.GA2353095@rfd.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/651d005e76bc0b9542615f609b4d0d946035dc58

Modified Files
--------------
contrib/dblink/expected/dblink.out        |  6 +++---
src/backend/utils/adt/datetime.c          |  5 +++--
src/interfaces/ecpg/pgtypeslib/interval.c |  5 +++--
src/interfaces/libpq/fe-print.c           |  4 ++--
src/test/regress/expected/interval.out    | 22 +++++++++++-----------
5 files changed, 22 insertions(+), 20 deletions(-)