Clarify how --portable behaves (location of datadir)

[SomberNight]

I find the behaviour of the --portable/-P flag confusing, and in some cases unintended.

Note that the "portable" Windows exe we distribute works by implicitly setting --portable. The user can also manually set the flag regardless of how they run Electrum.

For context, Electrum normally places its data directory (config file, wallet files, block headers, lightning gossip, etc) at ~/.electrum (or %APPDATA%/Electrum on Windows). The --portable flag is there to change this: if set, Electrum will instead create a folder named electrum_data at ... well it depends, but usually in $PWD (the current working directory).

The current behaviour is as follows:

• "pyinstaller" case: portable .exe, other .exes with --portable, and .dmg with --portable
  • uses $PWD
  • note that when you double-click the portable .exe on Windows, $PWD is set to the parent folder, i.e. the datadir gets put next to the .exe
• appimage --portable
  • broken (see Appimage Linux Electrum doesn't work with portable wallet - "read only filesystem" #5551)
• git clone
  • next to run_electrum
• unpacking tar.gz and running locally from it
  • next to run_electrum
• pip install *.tar.gz, and calling "electrum --portable" from terminal
  • uses python's user script directory
    • ~/.local/bin/electrum_data
    • $VIRTUAL_ENV/bin/electrum_data

I would like to consolidate this... but I realise some users might be relying on current behaviour.
I think the clean solution would be to simply always use $PWD, however, to minimise breakage, I suggest the following:

• "pyinstaller" case: portable .exe, other .exes with --portable, and .dmg with --portable
  • uses $PWD
• appimage --portable
  • (CHANGED) uses $PWD
• git clone
  • next to run_electrum
• unpacking tar.gz and running locally from it
  • next to run_electrum
• pip install *.tar.gz, and calling "electrum --portable" from terminal
  • (CHANGED) uses $PWD

That is, the only breaking change is re the last case, but I hope no one is relying on that ~madness.

[SomberNight]

Note: technically this is a breaking change but I think it is ok for it to get included in 4.2.1.
I doubt anyone was relying on behaviour that changed.