Make docs more consistent

Assorted nitpicking and OCD-ifying the docs. I have tried to find the most common convention and bring all functions into compliance. Specifically:

* Sentences should end in periods, never no punctation, and only colons if they directly refer to the code below.
* Code examples should be placed below the text, not inline in parentheses. A blank line should separate the two. Code blocks should be indented seven spaces (that's how I found them...).
* Verbs should be imperative ("Add an element") not descriptive ("Adds an element").
* The first sentence should lead with a strong, active verb.

This PR isn't meant to be comprehensive of all the docs, just a good chunk.

Evan has been busy, so would anyone else who is lurking please chime in and confirm I haven't screwed anything up?