completion: improve docs for activation

It was not clear if users should activate both sets of completions if they
wanted the dynamic ones.

The shell commands for activating the dynamic completions are aligned with the
static version. Users can choose / are responsible themselves to add the
activation to their shell config if they want them in every shell instance.

For fish specifically, `script | source` is more idiomatic than
`source (script | psub)`.

docs/install-and-setup.md

@@ -210,12 +210,13 @@ To set up command-line completion, source the output of
 `jj util completion bash/zsh/fish`. Exactly how to source it
 depends on your shell.
 
-Improved completions are currently in the works, these will complete things
-like bookmark names as well. You can activate them with the alternative "dynamic"
-instructions below. Please let us know if you encounter any issues, so we can
-ensure a smooth transition once we default to these new completions. If you
-have ideas about specific completions that could be added, please share them
-[here](https://github.com/martinvonz/jj/issues/4763).
+Improved completions are currently in the works, these will complete things like
+bookmark names as well. You can activate them with the alternative "dynamic"
+instructions below. They should still complete everything the static completions
+did, so only activate one of them. Please let us know if you encounter any
+issues, so we can ensure a smooth transition once we default to these new
+completions. If you have ideas about specific completions that could be added,
+please share them [here](https://github.com/martinvonz/jj/issues/4763).
 
 ### Bash
 
@@ -226,7 +227,7 @@ source <(jj util completion bash)
 dynamic:
 
 ```shell
-echo "source <(COMPLETE=bash jj)" >> ~/.bashrc
+source <(COMPLETE=bash jj)
 ```
 
 ### Zsh
@@ -240,7 +241,7 @@ source <(jj util completion zsh)
 dynamic:
 
 ```shell
-echo "source <(COMPLETE=zsh jj)" >> ~/.zshrc
+source <(COMPLETE=zsh jj)
 ```
 
 ### Fish
@@ -252,7 +253,7 @@ jj util completion fish | source
 dynamic:
 
 ```shell
-echo "source (COMPLETE=fish jj | psub)" >> ~/.config/fish/config.fish
+COMPLETE=fish jj | source
 ```
 
 ### Nushell