ui: Clarify help messages for 'message' fields

Add or amend tooltips to clarify what the message is for and that it
doesn't go over the network.

src/qt/forms/receivecoinsdialog.ui

@@ -46,21 +46,21 @@
      <item row="4" column="2">
       <widget class="QLineEdit" name="reqLabel">
        <property name="toolTip">
-        <string>The label to associate with the new receiving address</string>
+        <string>An optional label to associate with the new receiving address</string>
        </property>
       </widget>
      </item>
      <item row="6" column="2">
       <widget class="QLineEdit" name="reqMessage">
        <property name="toolTip">
-        <string>The message to attach to payment request</string>
+        <string>An optional message to attach to the payment request, which will be displayed when the request is opened. Note: The message will not be sent with the payment over the Bitcoin network.</string>
        </property>
       </widget>
      </item>
      <item row="2" column="2">
       <widget class="QLabel" name="label_5">
        <property name="text">
-        <string>Use this form to request payments. All fields are optional.</string>
+        <string>Use this form to request payments. All fields are &lt;b&gt;optional&lt;/b&gt;.</string>
        </property>
       </widget>
      </item>
@@ -99,7 +99,7 @@
         </size>
        </property>
        <property name="toolTip">
-        <string>The amount to request</string>
+        <string>An optional amount to request. Leave this empty or zero to not request a specific amount.</string>
        </property>
       </widget>
      </item>

src/qt/forms/sendcoinsentry.ui

@@ -153,6 +153,9 @@
     </item>
     <item row="3" column="1">
      <widget class="QLabel" name="messageTextLabel">
+      <property name="toolTip">
+       <string>A message that was attached to the Bitcoin URI which will be stored with the transaction for your reference. Note: This message will not be sent over the Bitcoin network.</string>
+      </property>
       <property name="textFormat">
        <enum>Qt::PlainText</enum>
       </property>