plugins: Updated doxygen for the new plugin loader

The documentation provides a quite detailed description of the new loader
In addition it adds a "how to transition" that briefly describes the old
loader (for curious newcomers) and lots of hints for porting legacy
plugins to the new loader.

doc/pluginsymbols.c

@@ -23,7 +23,12 @@
 
 /**
  * @file pluginsymbols.c
- * Symbols declared from within plugins.
+ * Symbols declared from within plugins, all of this is <b>deprecated</b>.
+ *
+ * @deprecated This is the legacy way of making plugins for Geany. Refer to @ref howto for the
+ * reworked process and @ref legacy to learn how to port your plugin to that new world.
+ * Meanwhile Geany will still load plugins programmed against this interface (even the items that
+ * are marked deprecated individually such as @ref plugin_fields).
  *
  * Geany looks for these symbols (arrays, pointers and functions) when initializing
  * plugins. Some of them are optional, i.e. they can be omitted; others are required
@@ -106,4 +111,3 @@ void plugin_cleanup();
  * or something else.
  * Can be omitted when not needed. */
 void plugin_help();
-

src/plugindata.h

@@ -273,9 +273,12 @@ void plugin_cleanup(void);
  * Then fill in GeanyPlugin::info and GeanyPlugin::funcs of the passed @p plugin. Finally
  * GEANY_PLUGIN_REGISTER() and specify a minimum supported API version.
  *
+ * For all glory details please read @ref howto.
+ *
  * @param plugin The unique plugin handle to your plugin. You must set some fields here.
  *
  * @since 1.26 (API 225)
+ * @see @ref howto
  */
 void geany_load_module(GeanyPlugin *plugin);
 
@@ -291,6 +294,7 @@ void geany_load_module(GeanyPlugin *plugin);
  * pointer.
  *
  * @since 1.26 (API 225)
+ * @see @ref howto
  **/
 struct GeanyPluginFuncs
 {
@@ -315,6 +319,7 @@ void geany_plugin_set_data(GeanyPlugin *plugin, gpointer data, GDestroyNotify de
  * It simply calls geany_plugin_register() with GEANY_API_VERSION and GEANY_ABI_VERSION.
  *
  * @since 1.26 (API 225)
+ * @see @ref howto
  **/
 #define GEANY_PLUGIN_REGISTER(plugin, min_api_version) \
 	geany_plugin_register((plugin), GEANY_API_VERSION, \