adjust docstrings for `_iter_shard` methods / functions

[melonora]

For all _iter_shard methods and functions there is a fallback to chunks when no shards are present, but this is not clear from either the name or the docstring. Though we can't quickly change the names, the docstrings could be more informative. In this PR I adjusted the docstrings.

Note: based on discussion with @d-v-b chunks technically do not have keys, but because of the fallback when shards are not present these are currently returned. Broader discussion will be whether chunks should also return keys.

The current situation stems from the ambiguity in the meanings of 'shard' and 'chunk'.

TODO:

• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

@joshmoore