Document new r.range command

[VeXocide]

rethinkdb/rethinkdb#875 which is currently in review adds a new r.range command which needs to be documented.

Three versions currently exist, first of all r.range() which generates an infinite sequence from 0 onwards. Secondly r.range(n) which produces a sequence from 0 to n, not including n, and r.range(n, m) which produces a sequence from n to m, including n but not including m.

[chipotle]

It looks like this is still a little in flux, with a step option coming?

[VeXocide]

There are two proposals at the moment, the first of which is rethinkdb/rethinkdb#3148 to add the step optarg to r.range(). This is fairly trivial for integers thus I'll probably end up adding that soon.

The second, rethinkdb/rethinkdb#3149, is to add support for more types, particularly dates and floating point numbers. It's more involved and will require some discussion, thus that'll probably take some more time.

[VeXocide]

Neither step nor support for more types will make it into 1.16 as they both need some discussion, thus it's just the r.range as initially described.