This adds `counter_long` and `counter_double` to the signatures of
supported fields for `IS NULL` and `IS NOT NULL`. We hadn't been
generating those signatures since the docs v3 migration, so this had to
plug those in. In addition, it changes the wording on a few things and
adds a note that if a field is only in some documents then the ones
missing the field will have `NULL` - which is important information for
`IS NULL` and `IS NOT NULL`.
59a10bdd7b