refactor: add info label for custom user agent field

src/app/home/recent-playlists/playlist-info/playlist-info.component.html

@@ -36,11 +36,14 @@
             }}</mat-label>
             <input matInput formControlName="count" />
         </mat-form-field>
-        <mat-form-field class="full-width">
+        <mat-form-field class="full-width mb-10">
             <mat-label>{{
                 'HOME.PLAYLISTS.INFO_DIALOG.USER_AGENT' | translate
             }}</mat-label>
             <input formControlName="userAgent" matInput />
+            <mat-hint>{{
+                'HOME.PLAYLISTS.INFO_DIALOG.CUSTOM_USER_AGENT' | translate
+            }}</mat-hint>
         </mat-form-field>
         <mat-form-field class="full-width" *ngIf="playlist.filePath">
             <mat-label>