Add docs related to zed/4758 changes

[philrz]

This is my attempt to update the docs to reflect what changed in #4758. This took longer than I thought because the flexibility of having -lake, multiple possible environment variables, and the new fallback default paths play with the different zed personalities in many ways. Thankfully the hints from the zed CLI seem to point the user in the right direction if they attempt a combo that's not intended to work, so I stopped short of trying to enumerate every combo and instead continue to lean on what was already written in the "personalities" section to fill in some of the detail (e.g., when URLs can be used vs. file system paths).

I'm expecting 99% of users will stay on the happy path, i.e., when they use zed in its client personality they won't have specified -lake or any environment variables but if their Zui is running it'll have started a lake service and it'll "just work". We do still need to make changes on the Zui side to make it "just work" in the other direction (i.e., so Zui will start defaulting to using storage at %LOCALAPPDATA%\zed on Windows or $HOME/.local/share/zed on Linux/macOS), but that'll be my next conquest once folks sign off on what's in this PR.

All that said, improvements welcomed!

[nwt]

Maybe tighten this?

Suggested change

	3. A Zed lake service running locally at `http://localhost:9867` (if a socket
	is listening at that port)
	3. A Zed lake service running at `http://localhost:9867`

[philrz]

@nwt: I actually kept this one un-tight intentionally. During black box testing I noticed that if I run a simple program that just listens on 9867 and accepts the incoming connection, and the lake location logic makes it to this step, zed effectively hangs indefinitely. I guess it's a hair-split to think that the text as I wrote it communicates this subtlety. But I was trying to express that all it takes is a listening socket for it to commit to this step.

#!/usr/local/bin/python3
import socket
import sys

if (len(sys.argv) != 2 or not sys.argv[1].isdigit()):
  print('Usage: listen <port>')
  exit()

p = int(sys.argv[1])
l = []
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(('', p))
s.listen(1)
while 1:
  (c, a) = s.accept()
  l.append(c)
  print('%d: connection from %s' % (len(l), a))

[nwt]

@philrz: That's a bug!

[philrz]

@nwt: Cool! Glad we discussed. I've just opened #4824 to track this.

[nwt]

Maybe tighten this?

Suggested change

	4. A `zed` subdirectory below a path in the
	[`XDG_DATA_HOME`](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html)
	environment variable (if defined)
	4. `$XDG_DATA_HOME/zed` (if the `XDG_DATA_HOME` environment variable is defined)

[philrz]

@nwt: This one was also intentionally un-tight because even on Windows the XDG_DATA_HOME environment variable may be obeyed. Obviously users have their choice of shell on Windows, and some use $ to expand. But since the out-of-the-box Command Prompt uses dual % to expand environment variables, I was keeping it higher level. Maybe another futile hair-split, though.