Docstring cleanup of physics/mechanics/body.py
Docstring cleanup of physics/mechanics/body.py

I made a number of changes in the docstrings of physics/mechanics/body.py

- I added clarity in the beginning paragraphs of the Body class as to
  when a RigidBody would be made as opposed to a Particle.
- Added an attributes section to Body's docstring
- Reordered the parameters in Body's docstring to match the input order
- Renamed the inertia parameter so that it matched the keyword argument
  for `__init__()`
- I revised the prose in the examples section of Body's docstring
- I revised the prose and some of the code in apply_force()'s docstring
    - It seems like the second example was copy pasted from the joints
      code and as such randomly had two Body's (parent and child) even
      though only one was used. Also there was no definition of a body
      variable in the second example even though it is used.
- I added an example to apply_torque()

There are also a handful of other smaller changes.

sampadsaha5/sympy,skidzo/sympy,postvakje/sympy,kaushik94/sympy,aktech/sympy,jaimahajan1997/sympy,souravsingh/sympy,sampadsaha5/sympy,aktech/sympy,drufat/sympy,yashsharan/sympy,madan96/sympy,skidzo/sympy,chaffra/sympy,drufat/sympy,yashsharan/sympy,aktech/sympy,souravsingh/sympy,drufat/sympy,postvakje/sympy,madan96/sympy,jaimahajan1997/sympy,madan96/sympy,skidzo/sympy,jaimahajan1997/sympy,postvakje/sympy,kaushik94/sympy,souravsingh/sympy,chaffra/sympy,chaffra/sympy,sampadsaha5/sympy,kaushik94/sympy,yashsharan/sympy