doc: Update instructions on using native-completions (#15480)

`~/.local/share/bash-completion/completions/cargo` is more sensible, and
[recommended by
rustup](https://github.com/rust-lang/rustup/blob/e4f3ad6f893e56ca10f0a329c401a5eafab69607/src/cli/help.rs#L275).

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Author: weihanglo

src/doc/src/reference/unstable.md

@@ -1817,10 +1817,10 @@ When in doubt, you can discuss this in [#14520](https://github.com/rust-lang/car
 
 ### How to use native-completions feature:
 - bash:
-  Add `source <(CARGO_COMPLETE=bash cargo +nightly)` to your .bashrc.
+  Add `source <(CARGO_COMPLETE=bash cargo +nightly)` to `$XDG_DATA_HOME/bash-completion/completions/cargo`, or `~/.local/share/bash-completion/completions/cargo` if `XDG_DATA_HOME` is not set.
 
 - zsh:
-  Add `source <(CARGO_COMPLETE=zsh cargo +nightly)` to your .zshrc.
+  Add `source <(CARGO_COMPLETE=zsh cargo +nightly)` to your `.zshrc`.
 
 - fish:
   Add `source (CARGO_COMPLETE=fish cargo +nightly | psub)` to `$XDG_CONFIG_HOME/fish/completions/cargo.fish`