Signal-desktop: Clarify Linux Deskop install instructions

[amandabee]

• [x] I have searched open and closed issues for duplicates

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The Linux Desktop instructions at https://support.signal.org/hc/en-us/articles/360007211952-Troubleshooting-Installs-or-Updates#update_desktop_troubleshooting include instructions for adding Signal to my apt sources but specify the release as Xenial. I (reasonably, I think) updated that to my current distro, Bionic, which didn't work. Some hunting around in the issues lead me to Issue #2521 which indicates that a different article https://support.signal.org/hc/en-us/articles/360008216551 once included instructions to hardcode the release to Xenial.

That worked.

There are a few issues that ask for a fix to the underlying problem (eg. #1730 and #2521) which would be great, but in the meantime,

• https://support.signal.org/hc/en-us/articles/360008216551 should include instructions or a link to instructions and
• https://support.signal.org/hc/en-us/articles/360007211952-Troubleshooting-Installs-or-Updates#update_desktop_troubleshooting should add a note such as "Use "Xenial," even if you are running Debian or an Ubuntu release newer than Xenial." to save folks from confusion in the interim.

[anoxi]

Links to the operations systems under Desktop here https://support.signal.org/hc/en-us/articles/360008216551 still point to the same page, so Linux Deskop install instructions can just be found under the somewhat hidden Trouleshooting page.

[Cubevoid]

Signal Desktop is also available as a Snap and Flatpak, which is a much easier option than installing via apt, and supports all distributions.

[alexsapps]

Any Crostini-specific instructions would be helpful for Chrome OS users as well.

[kenpowers-signal]

Please note that currently our only officially supported distribution channel for Linux is our APT repository as documented at https://signal.org/download/

[amandabee]

That documentation should be clear that everyone, even folks running bionic should use the xenial repo.

[LudwikJaniuk]

It seems that the first bullet point of this issue is resolved at this point in time, since https://support.signal.org/hc/en-us/articles/360008216551 does include a link to "Download and open Signal Desktop", which when you press "download" does show instructions for downloading (tell you to copy command lines).

The second bullet still stands, as one might indeed be tempted to change from "xenial".