Improve documentation for steam

[docwhat]

Explain the options you can pass into stream.

What to pass into match was causing me a lot of problems because I kept using site/css/**/*.css instead of just css/**/*.css; though I now realized that **/*.css would work fine too, since the stream only includes css files.

I wasn't sure to add something about .map files too...

[coveralls]

Coverage remained the same at 95.402% when pulling 0aa00a7 on docwhat:patch-1 into cbe2ccb on BrowserSync:master.

[coveralls]

Coverage remained the same at 95.402% when pulling 0aa00a7 on docwhat:patch-1 into cbe2ccb on BrowserSync:master.

[shakyShane]

Thanks!