Update UserDocumentation.rst

Add _LIBCPP_REMOVE_TRANSITIVE_INCLUDES to the list of configuration macros

Author: martinlicht

libcxx/docs/UserDocumentation.rst

@@ -128,6 +128,24 @@ enable or disable extended libc++ behavior.
   replacement scenarios from working, e.g. replacing `operator new` and
   expecting a non-replaced `operator new[]` to call the replaced `operator new`.
 
+**_LIBCPP_REMOVE_TRANSITIVE_INCLUDES**:
+  When this macro is defined, the standard library headers will adhere to a
+  stricter policy regarding the (transitive) inclusion of other standard library
+  headers, only guaranteeing to provide those definitions explicitly mandated by
+  the standard. Please notice that defining this macro might break any existing
+  codebase that implicitly relies on standard headers providing any definitions
+  not explicitly required by the standard.
+
+  The primary motivation for this configuration macro is to improve compilation
+  times. In most standard library implementations, header files include more
+  definitions than officially required because the implementation details give rise
+  to internal dependencies. The common practice is to have the standard headers
+  internally include other standard headers, but this generally results in
+  increased compilation overhead. This configuration option attempts to mitigate
+  this problem by avoiding such unnecessary inclusions. In addition, compiling
+  a codebase with this macro may improve maintainability by identifying
+  missing standard header inclusions.
+
 **_LIBCPP_DISABLE_DEPRECATION_WARNINGS**:
   This macro disables warnings when using deprecated components. For example,
   using `std::auto_ptr` when compiling in C++11 mode will normally trigger a