Deprecate alternate syntax for $maxDistance with $near

Deprecate with an error message that the alternate, undocumented $maxDistance syntax for $near.

Currently, the following syntax:

db.t.find( { loc : { $near : [56, 8, 10] } } )

is a synonym for this:

db.t.find( { loc : { $near : [56, 8], $maxDistance:10 } } )