docs: Document setting up command completion for logcli (grafana#7325)

This documents the existing command completion that kingpin provides
(docs
[here](https://github.com/alecthomas/kingpin#bashzsh-shell-completion)
on that).

**Which issue(s) this PR fixes**:
Fixes grafana#2949

CHANGELOG.md

@@ -55,6 +55,9 @@
 
 ##### Changes
 
+#### Logcli
+* [7325](https://github.com/grafana/loki/pull/7325) **dbirks**: Document setting up command completion
+
 #### Fluent Bit
 
 #### Loki Canary

docs/sources/tools/logcli.md

@@ -33,6 +33,20 @@ Optionally, move the binary into a directory that is part of your `$PATH`.
 cp cmd/logcli/logcli /usr/local/bin/logcli
 ```
 
+## Set up command completion
+
+You can set up tab-completion for `logcli` with one of the two options, depending on your shell:
+
+- For bash, add this to your `~/.bashrc` file:
+  ```
+  eval "$(logcli --completion-script-bash)"
+  ```
+
+- For zsh, add this to your `~/.zshrc` file:
+  ```
+  eval "$(logcli --completion-script-zsh)"
+  ```
+
 ## LogCLI usage
 
 ### Grafana Cloud example