docs: bookmarked: add comments to Bookmarked functions

[JGBSouza]

This pr add simple comments in the Bookmarked functions improving its documentation.

[lorenzoberts]

Hey @JGBSouza, thanks for the contribution.

1. I'm not sure we need to describe implementation details in the docstrings, specially for simple functions, which are fair easy to understand by looking at the implementation. So I would prefer if the function comments were more objective regarding the function purpose.
   Instead of "Add a patchset to the list of bookmarks. The patchset is only added if it is not already on the list.", you could simply say "Add a patchset to the list of bookmarks if it's not bookmarked". The functions are simple, so the docstring should be proportional to that.
2. Let me know if the "copy" comment makes sense, since it's specific because of Rust details it could be a little counterintuitive.
3. Regarding the commit message, it's the same thing as the previous PR. We just need 1 prefix, specially for contributions that are relatively simple

[lorenzoberts]

In Rust, "copy" probably has a different meaning compared to other languages. I understood what you meant, but it's not super precise in terms of Rust. Since you're clining the Patch, you're not returning a copy of it, but a clone, a.k.a deep copy, which duplicates the Patch in memory