Add better documentation for browser wasmFs (memFs)

[hlolli]

I've been struggling for two days getting preopens working with memFs. I know I'm in a very experimental territory so it's all good. But I still fail to see any example of creating file (via open/fopen/openat) in a browser environment. I keep getting System error : Capabilities insufficient. but if there would be just one example of this somewhere available, I could definitely get on track.

Stay healthy, cheers!

[sfranzyshen]

things get changed faster than they get documented around here ...