text: Title: [DOC] API Documentation for Chemistry functions is not standardized
  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 docs do not have a standard docstring format for functions. Some docstrings contain only `Parameters`. Some have `Parameters` and `Method chaining usage`.  Some have a combination of `Parameters`, ` Returns`, `Functional usage example`, and `Method chaining example` but not each one of these pieces.
  
  I would like to propose a change, such that now the docs contain a **standardized** docstring suite. All functions should contain (at a minimum) the following:
    - `Parameters`
    - ` Returns`
    - `Functional usage example`
    - `Method chaining example`
  
  **NOTE**: This can be done for all functions within the `janitor` directory. For ease of review, this will focus on the `chemistry.py` file and move to other files/functions as time permits.
  
  # 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. -->
  
  - [Link to documentation page](https://pyjanitor.readthedocs.io/reference/chemistry.html)
  - [Link to exact file to be edited](https://github.com/ericmjl/pyjanitor/blob/dev/janitor/chemistry.py)