text: Title: [DOC] Check that all functions have a `return` and `raises` section within docstrings
  Body: # Brief Description of Fix
  
  <!-- Please describe the fix in terms of a "before" and "after". In other words, what's not so good about the current docs
  page, and what you would like to see it become.
  
  Example starter wording is provided. -->
  
  Currently, the docstrings for some functions are lacking a `return` description and (where applicable) a `raises` description.
  
  I would like to propose a change, such that now the docstrings for all functions within pyjanitor have a valid `return` and (where applicable) valid `raises` statements.
  
  **Requires a look at all functions** (not just the provided examples below).
  
  # Relevant Context
  
  <!-- Please put here, in bullet points, links to the relevant docs page. A few starting template points are available
  to get you started. -->
  
  - [Good example of complete docstring - janitor.complete](https://ericmjl.github.io/pyjanitor/reference/janitor.functions/janitor.complete.html)
  
  - Examples for missing `returns`:
    - [Link to documentation page - finance.convert_currency](https://ericmjl.github.io/pyjanitor/reference/finance.html)
    - [Link to exact file to be edited - finance.py](https://github.com/ericmjl/pyjanitor/blob/dev/janitor/finance.py)
  
    - [Link to documentation page - functions.join_apply](https://ericmjl.github.io/pyjanitor/reference/janitor.functions/janitor.join_apply.html)
    - [Link to exact file to be edited - functions.py](https://github.com/ericmjl/pyjanitor/blob/dev/janitor/functions.py)
  