[doc] add note regarding C header inclusion to FFI documentation

docs/source/ffi/ffi.rst

@@ -29,6 +29,14 @@ the library. In this document, we will assume the default Chez Scheme code
 generator (the examples also work with the Racket or Gambit code generator) and
 that the foreign language is C.
 
+FFI declarations for C functions can also, optionally, take a C header file to
+be ``#include``'d in the C code generated by the Idris2 compiler. For instance:
+
+.. code-block:: idris
+
+    %foreign "C:idris2_resetRawMode, libidris2_support, idris_support.h"
+    idris2_resetRawMode : (1 x : %World) -> IORes ()
+
 Scheme Details
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