Fix arena flags

src/arena.h

@@ -5,12 +5,13 @@
 typedef U32 ArenaFlags;
 typedef struct ArenaOptions ArenaOptions;
 enum {
-    ArenaFlag_NoDecommit, // arena_ temp_end/reset/reset_to doesn't decommit memory
+    ArenaFlag_NoDecommit = 1, // operations which shrinks the arena does not decommit memory.
+                              // (arena_trim() can still decommit when called explicitly)
 };
 struct ArenaOptions {
     U32 size;
     U32 commit_size;
-    ArenaFlags flags; // No flags defined yet
+    ArenaFlags flags;
 };
 
 typedef struct MemoryBlock MemoryBlock;