Add doc-strings to the client_helpers

[maxmynter]

Add doc strings such that the client_helpers API are included in the documentation.

This addresses https://github.com/aai-institute/lakefs-spec/issues/121

The question remains open whether we want doc-strings (and hence documentation) on all the members where I added them. Some might not be intended as public API. Refer to the linked issue for more details.

[janwillemkl]

For the parameter descriptions, there is some inconsistency:

For example:

• The lakeFS client object
• Repository name
• A string representing the commit SHA

In principle it is also not needed to include the type in the description, because that information is already present in definition.

[AdrianoKF]

I took the liberty of answering a few of your questions by means of pushing a reformatting of the docstrings 😅 (without seeing your comments)

* What is the policy on the introductory sentence in the docstring re: linebreak after the multiline intro `"""`?

* Do we want to explicitly mention what the default is in argument descriptions, even if it's in the signature?

Arguable, but I lean towards omitting them (if the rendered docs show the defaults correctly - which it seems they do).

* What tense / grammatical case are we putting the synopses into? I would argue for imperative (example: `Merge branch xyz`) instead of 3rd person (`Merges branch xyz`).

I don't care either way, but agree with what the Google Python style guide has to say on the matter:

The docstring may be descriptive-style ("""Fetches rows from a Bigtable.""") or imperative-style ("""Fetch rows from a Bigtable."""), but the style should be consistent within a file.

* Do we give names and types to the return values in the `Returns` section? I think we just went with a one sentence-description so far (names are not really valuable anyway, since the user will assign the name by himself).

No names, IMO (in fact I edited them out)

---

A few general remarks, @maxmynter:

• Sections in the Numpy docstring style start unintended (e.g., the Notes block would otherwise render as a markdown code block instead)
• No colons after single line identifiers (e.g., return types, exceptions), these show up in the output otherwise

I also added a watch block to mkdocs.yml for the source folder to make editing docstrings easier (since mkdocs serve will automatically rebuild the docs when saving a changed source file).

[AdrianoKF]

Merging now, as these can always be improved later on.