Point to /usr/share/bash-completion/completions for system bash completions

/etc/bash_completion.d works too, but the downside is that everything
there is loaded eagerly at bash_completion load time, slowing down shell
startup.

From /usr/share/bash-completion/completions completion files are loaded
on demand. This mechanism is available as of bash-completion version 2
and later, just like the ~/.local/share/bash-completion/completions one
that is already suggested for user-specific completions.

Author: scop

src/cli/help.rs

@@ -169,7 +169,8 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
 
     Bash:
 
-    Completion files are commonly stored in `/etc/bash_completion.d/` for
+    Completion files are commonly stored in
+    `/usr/share/bash-completion/completions` for
     system-wide commands, but can be stored in
     `~/.local/share/bash-completion/completions` for user-specific commands.
     Run the command:

tests/suite/cli-ui/rustup/rustup_completions_cmd_help_flag_stdout.toml

@@ -26,7 +26,8 @@ Discussion:
 
     Bash:
 
-    Completion files are commonly stored in `/etc/bash_completion.d/` for
+    Completion files are commonly stored in
+    `/usr/share/bash-completion/completions` for
     system-wide commands, but can be stored in
     `~/.local/share/bash-completion/completions` for user-specific commands.
     Run the command: