Add tooltip to listening port spinbox

src/gui/optionsdialog.ui

@@ -1457,6 +1457,9 @@ Manual: Various torrent properties (e.g. save path) must be assigned manually</s
                  </item>
                  <item>
                   <widget class="QSpinBox" name="spinPort">
+                   <property name="toolTip">
+                    <string>Set to 0 to let your system pick an unused port</string>
+                   </property>
                    <property name="specialValueText">
                     <string>Any</string>
                    </property>

src/webui/www/private/views/preferences.html

@@ -231,7 +231,7 @@
         <legend>QBT_TR(Listening Port)QBT_TR[CONTEXT=OptionsDialog]</legend>
         <div class="formRow">
             <label for="port_value">QBT_TR(Port used for incoming connections:)QBT_TR[CONTEXT=OptionsDialog]</label>
-            <input type="text" id="port_value" style="width: 4em;" />
+            <input type="text" id="port_value" style="width: 4em;" title="QBT_TR(Set to 0 to let your system pick an unused port)QBT_TR[CONTEXT=OptionsDialog]" />
             <button style="margin-left: 1em;" onclick="qBittorrent.Preferences.generateRandomPort();">QBT_TR(Random)QBT_TR[CONTEXT=OptionsDialog]</button>
         </div>
         <div class="formRow">