Document mbedtls_mpi_uint and mbedtls_mpi_sint

Since they're part of the public API (even if only through a few functions),
they should be documented.

I deliberately skipped documenting how to configure the size of the type.
Right now, MBEDTLS_HAVE_INT32 and MBEDTLS_HAVE_INT64 have no Doxygen
documentation, so it's ambiguous whether they're part of the public API.
Resolving this ambiguity is out of scope of my current work.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/bignum.h

@@ -179,6 +179,20 @@
     #endif /* !MBEDTLS_NO_UDBL_DIVISION */
 #endif /* !MBEDTLS_HAVE_INT64 */
 
+/** \typedef mbedtls_mpi_uint
+ * \brief The type of machine digits in a bignum, called _limbs_.
+ *
+ * This is always an unsigned integer type with no padding bits. The size
+ * is platform-dependent.
+ */
+
+/** \typedef mbedtls_mpi_sint
+ * \brief The signed type corresponding to #mbedtls_mpi_uint.
+ *
+ * This is always an signed integer type with no padding bits. The size
+ * is platform-dependent.
+ */
+
 #ifdef __cplusplus
 extern "C" {
 #endif