Document Bash completion setup

docs/cli.md

@@ -72,7 +72,7 @@ Applications may install shell completion explicitly. When installed, generate
 and load the script for the executable name chosen by the application:
 
 ```sh
-# Bash
+# Bash, after initializing bash-completion
 source <(app completion bash)
 # Zsh, after initializing its completion system with compinit
 source <(app completion zsh)
@@ -80,6 +80,18 @@ source <(app completion zsh)
 app completion fish | source
 ```
 
+Bash requires the `bash-completion` package and its helpers in the current
+shell. If your startup files do not initialize it, source the package's
+initialization file before loading the generated script. For example, on
+systems that install it at the following path:
+
+```sh
+source /usr/share/bash-completion/bash_completion
+source <(app completion bash)
+```
+
+Use the path provided by your Bash completion installation.
+
 The application selects the executable basename at installation; use that same
 name to invoke the generated script. Completion offers canonical command and
 flag names, static values, and file or directory paths where declarations