Updates to "Multiprocess Firefox" docs on MDN

[William Bamberg]

Hi everyone

I've just added a couple of new pages to the documentation on multiprocess
Firefox:

* limitations of chrome scripts: things that you used to be able to do in
chrome code in single-process Firefox, that won't work any more in
multiprocess Firefox, with details of shims for incompatible add-ons.
https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox/Limitations_of_chrome_scripts

* limitations of frame scripts: things that you might expect to be able to
do in a frame script, that you can't, and what to do instead.
https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox/Limitations_of_frame_scripts

These pages came from a meeting with some of the e10s team, and from some
experimentation of my own. They have had a preliminary technical review.

They're really just a start, and I hope to be able to add to them as things
come up. So, a request: if you know of anything that's not listed here,
that seems worth adding, please add it or let me know so I can.

Even if it seems obvious to you that you can't use a particular interface
in the content process, or in the chrome process, it's probably worth
adding here.

Thanks!

Will