Add missing headings and remove table captions (#8892)

* Add missing headings and ToC anchors

Adds missing headings (and table of contents anchors) for 'Renamed Enum Values' and 'Renamed Methods' tables.

* Removed table captions

Which are redundant due to the headings that the tables sit under

doc/api_break.dox

@@ -61,7 +61,6 @@ Moved Classes        {#qgis_api_break_3_0_moved_classes}
 -------------
 
 <table>
-<caption id="moved_classes">Moved classes</caption>
 <tr><th>class<th>Module 2.X<th>Module 3.x
 <tr><td>QgsMapLayerModel<td>gui<td>core
 <tr><td>QgsMapLayerProxyModel<td>gui<td>core
@@ -72,7 +71,6 @@ Renamed Classes        {#qgis_api_break_3_0_renamed_classes}
 ---------------
 
 <table>
-<caption id="renamed_classes">Renamed classes</caption>
 <tr><th>API 2.x<th>API 3.X
 <tr><td>QgisGui<td>QgsGuiUtils
 <tr><td>QgsAbstractGeometryV2<td>QgsAbstractGeometry
@@ -230,15 +228,21 @@ Renamed Classes        {#qgis_api_break_3_0_renamed_classes}
 <tr><td>vertexData<td>QgsInterpolatorVertexData
 </table>
 
+
+Renamed Enum Values        {#qgis_api_break_3_0_renamed_enum_values}
+-------------------
+
 <table>
-<caption id="renamed_enum_values">Renamed enum values</caption>
 <tr><th>class</th><th>API 2.x<th>API 3.X
 <tr><td>QgsLayerTreeModelLegendNode<td>SymbolV2LegacyRuleKeyRole<td>SymbolLegacyRuleKeyRole
 <tr><td>QgsVectorLayer<td>EditorWidgetV2<td>EditorWidget
 </table>
 
+
+Renamed Methods        {#qgis_api_break_3_0_renamed_methods}
+---------------
+
 <table>
-<caption id="renamed_methods">Renamed method names</caption>
 <tr><th>class</th><th>API 2.x<th>API 3.X
 <tr><td>QgsAnnotation<td>mapPositionFixed<td>hasFixedMapPosition
 <tr><td>QgsApplication<td>defaultStyleV2Path<td>defaultStylePath