disk cache: add 2 commented out prefs for reference

[nodiscc]

Fixes item 1001 in #208

[pyllyukko]

I think this needs some additional researching and links in the user.js. Sounds like something that browser.cache.disk.enable already handles.

[nodiscc]

Added documentation

[pyllyukko]

Still... do we need these at all? Smart or dumb, the disk cache isn't in use and that can be verified from about:cache:

Number of entries: 	0
Maximum storage size: 	32768 KiB
Storage in use: 	0 KiB
Storage disk location: 	none, only stored in memory 

Of course there's the "When disk cache is enabled" note, but is it unnecessary clutter?

[nodiscc]

You are right. Closing, feel free to mark #208 item 1001 as fixed (#214)

[pyllyukko]

Done.