More explicit warning of restoring :all: programs

Closes: https://github.com/tmux-plugins/tmux-resurrect/issues/250

docs/restoring_programs.md

@@ -32,10 +32,18 @@ contains space-separated list of additional programs to restore.
 
         set -g @resurrect-processes 'false'
 
-- Restore **all** programs (be careful with this!):
+- Restore **all** programs (dangerous!):
 
         set -g @resurrect-processes ':all:'
 
+  Be *very careful* with this: tmux-resurrect can not know which programs take
+  which context, and a `sudo mkfs.vfat /dev/sdb` that was just formatting an
+  external USB stick could wipe your backup hard disk if that's what's attached
+  after rebooting.
+
+  This option is primarily useful for experimentation (e.g., to find out which
+  program is recognized in a pane).
+
 ### Clarifications <a name="clarfications"></a>
 
 > I don't understand tilde `~`, what is it and why is it used when restoring