Improve documentation in engine_const.inc (#665)

plugins/include/engine_const.inc

@@ -215,7 +215,12 @@ enum {
 	#endinput
 #endif
 #define _jghg_enums
+
+/**
+ * Used by get_global_[edict/float/int/string/vector]()
+ */
 enum {
+
 	// Edict
 	GL_trace_ent = 0,
 
@@ -260,6 +265,10 @@ enum {
 	GL_pSaveData
 };
 
+
+/**
+ * Used by [get/set]_usercmd()
+ */
 enum
 {
 	usercmd_float_start,
@@ -282,7 +291,9 @@ enum
 	usercmd_vec_end
 };
 
-// Used by the traceresult() native.
+/**
+ * Used by the traceresult()
+ */
 enum
 {
 	TR_AllSolid,       // (int) if true, plane is not valid