Explain `MRB_USE_MALLOC_TRIM`; ref #5069

doc/mruby3.md

@@ -96,6 +96,11 @@ does not fit in `Fixnum` integers.
 Pack `mrb_value` in a floating pointer number. Nothing
 changed from previous versions.
 
+== `MRB_USE_MALLOC_TRIM`
+
+Call `malloc_trim(0)` from mrb_full_gc() if this macro is defined.
+If you are using glibc malloc, this macro could reduce memory consumption.
+
 = Internal Changes
 
 == `Random` now use `xoshiro128++`.

include/mrbconf.h

@@ -101,6 +101,9 @@
 # endif
 #endif
 
+/* call malloc_trim(0) from mrb_full_gc() */
+//#define MRB_USE_MALLOC_TRIM
+
 /* string class to handle UTF-8 encoding */
 //#define MRB_UTF8_STRING