Make "modern" bash completion install recipes more dynamic

The downside of installing completions in their expanded form is that
they should be re-installed on respective tool updates to keep them in
sync. Generating on demand fixes that, in exchange for a few
milliseconds on first load.

A caveat is that the use of `<( ... )` requires the shell to be in
non-POSIX mode. In my opinion, POSIX mode interactive bash is too rare a
creature to pass on this recipe improvement.

Author: scop

Diff for: doc/user-guide/src/installation/index.md

@@ -73,10 +73,10 @@ simple as using one of the following:
 
 ```console
 # Bash
-$ rustup completions bash > ~/.local/share/bash-completion/completions/rustup
+$ printf '. <(rustup completions bash)\n' > ~/.local/share/bash-completion/completions/rustup
 
 # Bash (macOS/Homebrew)
-$ rustup completions bash > $(brew --prefix)/etc/bash_completion.d/rustup.bash-completion
+$ printf '. <(rustup completions bash)\n' > $(brew --prefix)/etc/bash_completion.d/rustup.bash-completion
 
 # Fish
 $ mkdir -p ~/.config/fish/completions

Diff for: src/cli/help.rs

@@ -175,7 +175,7 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
     Run the command:
 
         $ mkdir -p ~/.local/share/bash-completion/completions
-        $ rustup completions bash > ~/.local/share/bash-completion/completions/rustup
+        $ printf '. <(rustup completions bash)\n' > ~/.local/share/bash-completion/completions/rustup
 
     This installs the completion script. You may have to log out and
     log back in to your shell session for the changes to take effect.
@@ -186,7 +186,7 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
     With the `bash-completion` brew formula installed, run the command:
 
         $ mkdir -p $(brew --prefix)/etc/bash_completion.d
-        $ rustup completions bash > $(brew --prefix)/etc/bash_completion.d/rustup.bash-completion
+        $ printf '. <(rustup completions bash)\n' > $(brew --prefix)/etc/bash_completion.d/rustup.bash-completion
 
     Fish:
 
@@ -270,7 +270,7 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
 
     Bash:
 
-        $ rustup completions bash cargo > ~/.local/share/bash-completion/completions/cargo
+        $ printf '. <(rustup completions bash cargo)\n' > ~/.local/share/bash-completion/completions/cargo
 
     Zsh: