Make current documentation more verbose

[lcolladotor]

Several functions in biocthis are sparsely documented and refer users to documentation from other packages such as usethis. This is also noticeable on the main user guide at https://lcolladotor.github.io/biocthis/articles/biocthis.html.

We plan to expand the documentation and be more verbose such that users will have understand biocthis functions without having to go to other sources of documentation. Of course, we will still refer to external sources of documentation such that users can dive into further details if they want to do so. But the main idea should still be documented in biocthis.