Comprehensive docstrings for all top-level exports

[d-v-b]

Very few of the top-level functions have doctrings. This is bad and we should fix it, either by writing good docstrings from scratch or dynamically copying docstrings from other functions where appropriate.

[jhamman]

Huh! Most of the top level API functions should have docstrings. We copy these from the asynchronous API:

zarr-python/src/zarr/api/synchronous.py

Lines 292 to 316 in 498cb78

	consolidate_metadata.__doc__ = async_api.copy.__doc__
	copy.__doc__ = async_api.copy.__doc__
	copy_all.__doc__ = async_api.copy_all.__doc__
	copy_store.__doc__ = async_api.copy_store.__doc__
	load.__doc__ = async_api.load.__doc__
	open.__doc__ = async_api.open.__doc__
	open_consolidated.__doc__ = async_api.open_consolidated.__doc__
	save.__doc__ = async_api.save.__doc__
	save_array.__doc__ = async_api.save_array.__doc__
	save_group.__doc__ = async_api.save_group.__doc__
	tree.__doc__ = async_api.tree.__doc__
	array.__doc__ = async_api.array.__doc__
	group.__doc__ = async_api.group.__doc__
	open_group.__doc__ = async_api.open_group.__doc__
	create.__doc__ = async_api.create.__doc__
	empty.__doc__ = async_api.empty.__doc__
	empty_like.__doc__ = async_api.empty_like.__doc__
	full.__doc__ = async_api.full.__doc__
	full_like.__doc__ = async_api.full_like.__doc__
	ones.__doc__ = async_api.ones.__doc__
	ones_like.__doc__ = async_api.ones_like.__doc__
	open_array.__doc__ = async_api.open_array.__doc__
	open_like.__doc__ = async_api.open_like.__doc__
	zeros.__doc__ = async_api.zeros.__doc__
	zeros_like.__doc__ = async_api.zeros_like.__doc__

But looking at the docs page, this doesn't seem to be working as expected. Let's definitely fix this.