[zsh-completion] Add comment clarifying trigger expansion. (#230)

shell/completion.zsh

@@ -109,6 +109,8 @@ fzf-zsh-completion() {
   fi
 
   cmd=${tokens[1]}
+
+  # Explicitly allow for empty trigger.
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
 
   # Trigger sequence given