Docs/add doc to simulate data

[signekb]

Description

• This PR adds documentation (i.e., roxygen2 comments) to each function in simulate-data.R.
• This isn't technically necessary, but nice to help my understanding of what is going on in the functions created to simulate the data.
• I have moved add_fake_drug_name() from the section "Simulate data", since I don't believe it fits there? However, I moved it to "Insert extra values to overrepresent certain values", which might not be a good location either.
• I noticed that on L414, dplyr::group_split() is used which is lifecycle: experimental. Maybe we should use an alternative or is that fine?

[lwjohnst86]

Related to your question about group_split(). That function has been in experimental state for the last 4 years, so I don't expect it to disappear anytime soon. If these functions were critical to the package, I would change it, but it isn't critical.