``find_multiple_embeddings()`` needs to be added to docs

[JoelPasvolsky]

Description
The ParallelEmbeddingComposite composite of dwave-system states "the function find_multiple_embeddings() is called by default" so this function is documented there but not added in minorminer.

Expected behavior
Given that a documented composite is using it, we should add utils.parallel_embedings.find_multiple_embeddings function to this repo's documentation.

Additional context
Might want to see if other utility functions need to be added too.

[JoelPasvolsky]

Same for the utils.parallel_embedings.find_sublattice_embeddings function ("and find_sublattice_embeddings can be considered" also in the ParallelEmbeddingComposite class description).