Inform readers that account_command must be defined before remote access

[mervn]

For account_command to work as expected, it must be defined before accessing a host - via folder, smtp_url, etc.

This interaction is not noted on /feature/account-cmd but it would be very helpful to include a tip here. I am willing to write the tip, but am unsure if there is a particular format - like a TIP: prefix.

[flatcap]

Sorry, @mervn.
This got overlooked, then Christmas happened.

If you've still got the time to add some helpful text, great!

The docs for the Features are in docs/manual.xml.head in the neomutt/neomutt repository.

They're in DocBook XML format, but it's fairly obvious how that works.

https://github.com/neomutt/neomutt/blob/main/docs/manual.xml.head#L13182

If you have any questions just ask -- either here or on IRC.
IRC: #neomutt on irc.libera.chat (web client)

Thanks ❤️