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

contrib/dblink/expected/dblink.out

@@ -1099,9 +1099,9 @@ SELECT *
 FROM dblink('myconn',
     'SELECT * FROM (VALUES (''-1 2:03:04'')) i')
   AS i(i interval);
-        i         
-------------------
- -1 day -02:03:04
+         i         
+-------------------
+ -1 days -02:03:04
 (1 row)
 
 -- Try swapping to another format to ensure the GUCs are tracked