docs: Document the zsh-autocomplete partial-load error and fixes

Adds a Troubleshooting entry (symptom, cause, drop-or-keep fixes). The
keep path removes grml-zsh-config instead of setting
skip_global_compinit=1: grml's /etc/zsh/zshrc runs compinit
unconditionally and ignores that Debian/Ubuntu variable.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

airootfs/root/.zshrc-skel

@@ -7,8 +7,9 @@ zstyle ':omz:update' mode reminder # never auto-pull on startup
 
 # Plugins are pre-cloned into $ZSH/custom/plugins at ISO build time.
 # zsh-syntax-highlighting must load before zsh-history-substring-search.
-# zsh-autocomplete is cloned too but left off: it fights grml's compinit
-# and zsh-autosuggestions - add it here only if you drop those.
+# zsh-autocomplete is cloned too but left off: it must load before compinit,
+# which grml's /etc/zsh/zshrc already ran, and it fights zsh-autosuggestions.
+# To use it anyway, see README.md "zsh-autocomplete partial-load error".
 plugins=(
   git
   fzf-zsh-plugin