Fix grammar, spelling and formatting for new `HTML forms` docs

timgraham already feels that there is over-use of commas... I think that typically we don't use them enough to separate different thoughts in our documentation, but I don't think that using them to introduce pauses around phrases like "by contrast" is necessary.

My main concern here is make the reading and therefore comprehension as easy as possible to someone who might not be a native English speaker, which is why sometimes the most aesthetic or stylistically felicitous option is not the best.