Improve docs, and make the variable names consistent

Expanded most of the documentation, and renamed some of the variables
so that variables that are mutated are (almost) always called dst.
Update docs, remove change to Dot

Changed docs after feedback.

 Also got rid of code change to Dot, which should happen elsewhere.
remove extra blank line

don’t know how it snuck in
Merge pull request #10 from gonum/improve-docs-and-unify-vars

Improve docs, and make the variable names consistent