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

**Description**
The [ParallelEmbeddingComposite](https://docs.dwavequantum.com/en/latest/ocean/api_ref_system/composites.html#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.