Link include_directories to implicit_include_directories

docs/markdown/Include-directories.md

@@ -27,4 +27,6 @@ proper compiler flags to make it all work.
 
 Another thing to note is that `include_directories` adds both the
 source directory and corresponding build directory to include path, so
-you don't have to care.
+you don't have to care. If it turns out you don't want it after all, this can
+be disabled with the `implicit_include_directories` argument to the [build
+function](Reference-manual_functions.md) you use.