bdlcc_FixedQueue: update constructor documentation
bdlcc_FixedQueue: update constructor documentation

Clarify the restriction placed on the supplied 'capacity' in
bdlcc_FixedQueue's constructor.

bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,che2/bde,che2/bde,che2/bde,bloomberg/bde,bloomberg/bde