Document "portable wallet"

[jidanni]

On Debian
$ man electrum

        -P, --portable
              portable wallet

$ electrum -h

      -P, --portable        Use local 'electrum_data' directory

User have no further clue as to what this is about.
Users eventually might find something if they Google hard.

So please add a 'portable wallet' section to the documents.

Say if this is a legacy feature, a current feature, a Windows Only
feature, a feature that means without it if CPU failure, disks containing the
wallet cannot be moved to another machine. Etc. Etc. Thanks.