Clarify comments in Makefile on -DLUAJIT_ENABLE_LUA52COMPAT.

src/Makefile

@@ -96,9 +96,10 @@ XCFLAGS=
 # make use of it.
 #XCFLAGS+= -DLUAJIT_DISABLE_FFI
 #
-# Enable some upwards-compatible features from Lua 5.2 that are unlikely
-# to break existing code (e.g. __pairs). Note that this does not provide
-# full compatibility with Lua 5.2 at this time.
+# Features from Lua 5.2 that are unlikely to break existing code are
+# enabled by default. Some other features that *might* break some existing
+# code (e.g. __pairs or os.execute() return values) can be enabled here.
+# Note: this does not provide full compatibility with Lua 5.2 at this time.
 #XCFLAGS+= -DLUAJIT_ENABLE_LUA52COMPAT
 #
 # Disable the JIT compiler, i.e. turn LuaJIT into a pure interpreter.