[SimonCropp]

Phase 4c of the cross-platform tray: the docs. docs/tray.md described a Windows tray, and docs/viewer.md said the tray's viewer is found on Windows only. Both have been out of date since the macOS and Linux heads (#989, #991), the per-RID packages (#998) and the update on every head (#1000).

docs/tray.md

• The intro says where the icon is on each platform.
• Installation says the one command fetches the package for the machine's platform, and what that package carries.
• Running points at "Run at startup" for each platform.
• Update is a new section, for every head: where it is offered, the three steps of the shell, why the tray's own dotnet is used, and that on Windows it stops a viewer the tray opened. A viewer holding the queue loses it unstaged then.
• On macOS and Linux is a new section:
  • the menu's fixed items;
  • the menu bar on macOS, and StatusNotifierItem on Linux, including GNOME needing an AppIndicator extension and what happens with nowhere to show the menu;
  • desktop notifications in place of balloons;
  • where "Run at startup" writes;
  • what is not there: the Options window, the hot keys, purge and the locked files prompt. "Open on left" and "Max instances to launch" are the environment variables the Options window sets on Windows.
• Locked files, Purge verified files and Options are marked Windows only, and Debug view says where the report goes on the other two.
• The logging directory has its macOS and Linux path.

docs/viewer.md

• The "Which copy runs" flowchart no longer marks the tray's copy as Windows only.
• The line after it says the tray carries a viewer on all three platforms.

The generated docs/*.md come from a Release build, which also checks the content rules. It reported 0 errors.