Add Docs to Adapter Transforms

[eodole]

Added documentation for the following transforms:

• keep
• to_array
• convert dtype
• ad_set: gave an explaination but no useage/example as I am unsure what exactly this transform is doing
• constrain: there are many method strings that all map to the same function so I only included the first/second argument

[stefanradev93]

Hey Leona, thanks for the docs! Can you ensure that the linter is passing by including the pre-commit hooks?

[paul-buerkner]

Thank you for adding some docs! I made some minor edits and ran the linter. Will merge now.