Add documentation for `zarr.api.asynchronous`

[sanketverma1704]

xref: #2215

This issue tracks the documentation progress for the zarr.api.asynchronous module. We'll be focusing on adding docstrings and code samples to the functions.

Progress:

• docstrings
• code samples

[sanketverma1704]

AFAIK the zarr.api.asynchronous module is fully documented, only leaving the internal methods.

Safe to close this @jhamman?