doc: Add a long description for version macros

This makes the docs a little less incomplete.

common/flatpak-version-macros.h.in

@@ -25,6 +25,13 @@
 #ifndef __FLATPAK_VERSION_MACROS_H__
 #define __FLATPAK_VERSION_MACROS_H__
 
+/**
+ * SECTION:flatpak-version-macros:
+ * @title: Version information
+ *
+ * These macros can be used to check the library version in use.
+ */
+
 /**
  * FLATPAK_MAJOR_VERSION:
  *