docker: document option-stacking setting

Fixes #9266

plugins/docker/README.md

@@ -10,3 +10,25 @@ plugins=(... docker)
 
 A copy of the completion script from the docker/cli git repo:
 https://github.com/docker/cli/blob/master/contrib/completion/zsh/_docker
+
+## Settings
+
+By default, the completion doesn't allow option-stacking, meaning if you try to
+complete `docker run -it <TAB>` it won't work, because you're _stacking_ the
+`-i` and `-t` options.
+
+[You can enable it](https://github.com/docker/cli/commit/b10fb43048) by **adding
+the lines below to your zshrc file**, but be aware of the side effects:
+
+> This enables Zsh to understand commands like `docker run -it
+> ubuntu`. However, by enabling this, this also makes Zsh complete
+> `docker run -u<tab>` with `docker run -uapprox` which is not valid. The
+> users have to put the space or the equal sign themselves before trying
+> to complete.
+>
+> Therefore, this behavior is disabled by default. To enable it:
+>
+> ```
+> zstyle ':completion:*:*:docker:*' option-stacking yes
+> zstyle ':completion:*:*:docker-*:*' option-stacking yes
+> ```