Improved TaleaRhythmMaker docs.

Added detailed example documenting how TaleaRhythmMaker.secondary_divisions
works.

Secondary divisions impose a cyclic split operation on the divisions
talea rhtyhm-makers produce.

(The secondary_divisions property might better be named something like
'split_divisions_every'.)

Secondary divisions interact with prolation addenda in an interesting way. The
doc string added in the this commit illustrates this point.