Paramtype2: Add missing type CPT2_GLASSLIKE_LIQUID_LEVEL

Add the missing paramtype2 for param2 controlling the liquid level
inside the glasslike_framed drawtype.
Add missing documentation of the feature to lua_api.txt.
Update and improve comments for drawtype enumerations in nodedef.h.

src/script/cpp_api/s_node.cpp

@@ -62,6 +62,7 @@ struct EnumString ScriptApiNode::es_ContentParamType2[] =
 		{CPT2_COLOR, "color"},
 		{CPT2_COLORED_FACEDIR, "colorfacedir"},
 		{CPT2_COLORED_WALLMOUNTED, "colorwallmounted"},
+		{CPT2_GLASSLIKE_LIQUID_LEVEL, "glasslikeliquidlevel"},
 		{0, NULL},
 	};