Add entry about cfg!(overflow_checks)

Related to rust-lang/rust#111096

Author: AngelicosPhosphoros

src/conditional-compilation.md

@@ -222,6 +222,10 @@ This can be used to enable extra debugging code in development but not in
 production.  For example, it controls the behavior of the standard library's
 [`debug_assert!`] macro.
 
+### `overflow_checks`
+
+Enabled when program compiled with overflow checks enabled, e.g. in development builds.
+
 ### `proc_macro`
 
 Set when the crate being compiled is being compiled with the `proc_macro`