Rename and document new config var

src/widgets.zsh

@@ -25,7 +25,7 @@ _zsh_autosuggest_modify() {
 	# Get a new suggestion if the buffer is not empty after modification
 	local suggestion
 	if [ $#BUFFER -gt 0 ]; then
-		if [ -z "$ZSH_BUFFER_MAX_SIZE" -o $#BUFFER -lt "$ZSH_BUFFER_MAX_SIZE" ]; then
+		if [ -z "$ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE" -o $#BUFFER -lt "$ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE" ]; then
 			suggestion="$(_zsh_autosuggest_suggestion "$BUFFER")"
 		fi
 	fi