Update docs to show prepend_bos and padding_side are optional

[UFO-101]

Description

• Improve documentation for prepend_bos and padding_side in several functions in HookedTransformer and utils. It is currently unclear that they are just intended to be used as overrides of the default values set in the initial config.
• Improve the syntax of some type hints.

Type of change

• This change requires a documentation update

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[alan-cooney]

I think a better approach may be to just make this clear in the docstring, so that we don't have to release a breaking change. As long as the title is e.g. "Override Default for Prepend BOS" it's fairly clear.

The other changes seem good but mind splitting into separate PRs so we can keep the commit history clean?

[UFO-101]

@alan-cooney Undid parameter renaming and added / updated docstrings instead.

Moved other changes into new PRs #420 and #421

[neelnanda-io]

+1 to not rename the parameters, and just update the docs

[alan-cooney]

We're going to add some more standards for docstrings shortly, but approving now and will make some tweaks once those are added.