installer: improve existing rc backup nag

[abathur]

While looking into #5521 I noticed that the installer instructions don't really connect the dots about what the user is expected to do.

This is a tentative rephrase, but I also realize that this kind of thing may well be informed by history--that we're intentionally leaving out steps so that the kinds of people confused by this will ask for help instead of screwing up their systems and trashing us on social. I think GC would've cautioned if this were the case.

Objections? @toonn @grahamc @domenkozar

for reference, should look something like:

I back up shell profile/rc scripts before I add Nix to them. 
I need to back up /etc/bashrc to /etc/bashrc.backup-before-nix,
but the latter already exists.

Here's how to clean up the old backup file:

1. Back up (copy) /etc/bashrc and /etc/bashrc.backup-before-nix to
   another location, just in case.

2. Ensure /etc/bashrc.backup-before-nix does not have anything
   Nix-related in it. If it does, something is probably quite
   wrong. Please open an issue or get in touch immediately.

3. Once you confirm /etc/bashrc is backed up and doesn't already
   mention Nix, replace it with /etc/bashrc.backup-before-nix.

[abathur]

I would, to be frank, like it to work as commented, and not as coded.

I didn't want to let that pass completely undocumented--but I'm chasing other white whales at the moment and do not have the emotional/intellectual energy to make this happen without an installer test suite that ensures the important bits stay nailed down.

[grahamc]

I left some feedback with abathur in a PM on Matrix about point #3 and rewording it a bit, but this looks great to me.

[abathur]

I left some feedback with abathur in a PM on Matrix about point #3 and rewording it a bit, but this looks great to me.

I've force-pushed addressing feedback per 3 :)