docs(mango): match description of `$mod` with reality

The remainder argument for the `$mod` operator can be zero, while
its documentation suggests otherwise.  It actually covers a very
realistic use case where divisibility is expressed.

Neither related restrictions could be identified in the sources
[1] nor MongoDB forbids this [2].  Tests also seem to exercise this
specific case [3].  Thanks @iilyak for checking on these.

[1] https://github.com/apache/couchdb/blob/adf17140e81d0b74f2b2ecdea48fc4f702832eaf/src/mango/src/mango_selector.erl#L512:L513
[2] https://www.mongodb.com/docs/manual/reference/operator/query/mod/
[3] https://github.com/apache/couchdb/blob/0059b8f90e58e10b199a4b768a06a762d12a30d3/src/mango/test/03-operator-test.py#L58
2 files changed