text: Title: [DOC] Follow PEP 257 Docstring conventions
  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.
  
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  I propose that we follow the docstring conventions laid out in [PEP 257](https://www.python.org/dev/peps/pep-0257/).
  
  Namely:
  >One-liners are for really obvious cases. They should really fit on one line. For example:
  ```python
  def kos_root():
      """Return the pathname of the KOS root directory."""
      global _kos_root
      if _kos_root: return _kos_root
      ...
  ```
  and:
  >Multi-line docstrings consist of a summary line just like a one-line docstring, followed by a blank line, followed by a more elaborate description.
  ```python
  def complex(real=0.0, imag=0.0):
      """Form a complex number.
  
      Keyword arguments:
      real -- the real part (default 0.0)
      imag -- the imaginary part (default 0.0)
      """
      if imag == 0.0 and real == 0.0:
          return complex_zero
      ...
  ```
  
  We're using Sphinx, so the docstring will have a different body, but we can still follow the formatting.
  
  Following `pydocstyle`, the first line of the docstring should end in a period and be in imperative mood:
  ```python
  D401: First line should be in imperative mood: 'Do', not 'Does'.
  
  [Docstring] prescribes the function or method's effect as a command:
  ("Do this", "Return that"), not as a description; e.g. don't write "Returns the pathname ...".
  ```
  
  Additionally, all tests should have a one line docstring that explains what the test does (for everyone's sake), and what it is testing for. We don't actually have this in the docs, but it is part of my ongoing campaign with #306 
  
  The PR for this issue (which I am happy to do), will involve updating the docs to reflect these standards, and updating docstrings throughout the project.
  
  
  # 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://ericmjl.github.io/pyjanitor/CONTRIBUTION_TYPES.html)
  - [Link to exact file to be edited](https://github.com/ericmjl/pyjanitor/blob/dev/docs/CONTRIBUTION_TYPES.rst)
  