Expand sentence structure docs for doc comments

Third person singular present indicative form is, I believe, the
dominant form used in modern Rust documentation.
Tweak the style documentation on markdown

The markdown that Rustdoc supports is very close to GitHub Flavored
Markdown, which includes a number of extensions on top of the original
Markdown language.
Merge pull request #25 from kballard/doc-comment-formatting

Update style documentation for doc comments