Fix documentation

[sckott]

Hi @DASpringate Seems like many of the helper functions should changed in one of two ways. 1) remove all roxygen style docs and don't export them, or 2) use roxygen style docs, but if you do, you have to document parameter names, etc. or check isn't happy.

For example, in rpubmed_fetch file you have this function https://github.com/ropensci/rpubmed/blob/master/R/rpubmed_fetch.R#L59-L62 that has some roxygen docs, which then creates a .Rd file in the man/ directory, but because parameters aren't documented, check throws errors.

Do you want these helper functions internal only? Are they not supposed to be avail to users?