Add some docstring to prevent warning messages

- Some coding in tools.R giving more details to function
- New *.Rd files generated from running roxygen2::roxygenise()

/cc @mkcor 
The return of `show_credentials_file()` is actually the same as that of `get_credentials_file` (i.e., `List of keyword-value pairs (credentials)`).
You can compare:
```
ret <- show_credentials_file()
ret2 <- plotly:::get_credentials_file()
```