Document the GDNative singleton method prefix being changeable

This also tweaks error messages to remove hardcoded references to
the default `godot_` prefix.

modules/gdnative/doc_classes/GDNativeLibrary.xml

@@ -39,7 +39,7 @@
 			[b]Note:[/b] If the library defines tool scripts that run inside the editor, [code]reloadable[/code] must be [code]false[/code]. Otherwise, the editor will attempt to unload the tool scripts while they're in use and crash.
 		</member>
 		<member name="singleton" type="bool" setter="set_singleton" getter="is_singleton" default="false">
-			If [code]true[/code], Godot loads the library at startup rather than the first time a script uses the library, calling [code]gdnative_singleton[/code] after initializing the library. The library remains loaded as long as Godot is running.
+			If [code]true[/code], Godot loads the library at startup rather than the first time a script uses the library, calling [code]{prefix}gdnative_singleton[/code] after initializing the library (where [code]{prefix}[/code] is the value of [member symbol_prefix]). The library remains loaded as long as Godot is running.
 			[b]Note:[/b] A singleton library cannot be [member reloadable].
 		</member>
 		<member name="symbol_prefix" type="String" setter="set_symbol_prefix" getter="get_symbol_prefix" default="&quot;godot_&quot;">

modules/gdnative/gdnative.cpp

@@ -339,7 +339,7 @@ bool GDNative::initialize() {
 	if (err || !library_init) {
 		OS::get_singleton()->close_dynamic_library(native_handle);
 		native_handle = NULL;
-		ERR_PRINT("Failed to obtain godot_gdnative_init symbol");
+		ERR_PRINTS("Failed to obtain " + library->get_symbol_prefix() + "gdnative_init symbol");
 		return false;
 	}
 

modules/gdnative/register_types.cpp

@@ -277,7 +277,7 @@ void register_gdnative_types() {
 				proc_ptr);
 
 		if (err != OK) {
-			ERR_PRINT((String("No godot_gdnative_singleton in \"" + singleton->get_library()->get_current_library_path()) + "\" found").utf8().get_data());
+			ERR_PRINTS("No " + lib->get_symbol_prefix() + "gdnative_singleton in \"" + singleton->get_library()->get_current_library_path() + "\" found");
 		} else {
 			singleton_gdnatives.push_back(singleton);
 			((void (*)())proc_ptr)();