Add explainer for SD prompt expansion

public/scripts/extensions/stable-diffusion/settings.html

@@ -16,6 +16,10 @@
                 <input id="sd_expand" type="checkbox" />
                 Auto-enhance prompts
             </label>
+            <small>
+                This option uses an additional GPT-2 text generation model to add more details to the prompt generated by the main API.
+                Works best for SDXL image models. May not work well with other models, it is recommended to manually edit prompts in this case.
+            </small>
             <label for="sd_source">Source</label>
             <select id="sd_source">
                 <option value="extras">Extras API (local / remote)</option>