doc: Misc. improvements to nvim-related docs

- Improved wording in a few places for clarity
- Various capitalization/grammar fixes
- Change references to Neovim as 'editor' to 'Nvim'
- Be consistent regarding utilization of vim's documentation features,
  e.g. unnamedclip -> |unnamedclip|
- Reflowed all changed paragraphs accordingly

- Add spaces before parentheses
- Remove trailing whitespace
- Standardize single spaces after periods. Vim's docs use two for the
  most part, but Nvim's use one mainly, so just follow Nvim's conventions

runtime/doc/nvim_python.txt

@@ -13,16 +13,16 @@ Python plugins and scripting in Nvim				  *nvim-python*
 1. Introduction						    *nvim-python-intro*
 
 Through an external python interpreter connected via |msgpack-rpc|, Nvim
-offers some support for the classic |python-vim| interface. For now only the
+offers some support for the legacy |python-vim| interface. For now only the
 old Vim 7.3 API is supported.
 
 ==============================================================================
 2. Quickstart					       *nvim-python-quickstart*
 
-If you just want to start using python plugins with Nvim quickly, here's a
-simple step-by-step:
+If you just want to start using Vim python plugins with Nvim quickly, here's a
+simple tutorial:
 
-- Make sure python 2.6 or 2.7 is available on your `$PATH`
+- Make sure python 2.6 or 2.7 is available in your `$PATH`
 - Install the `neovim` python package:
   >
       $ pip install neovim