Spelling and grammar and stuff.

This mostly fixes some typos, and minor grammatical things.

Though when I got to the matrix stuff, I found it was an *indecipherable mess* ;). So I modified the markdown parser so that it would understand display math on multiple lines. That way the unreadable LaTeX matrices-on-a-single-line could be split apart for easier reading. So the diff looks larger than it is because I split those lines.