Update documentation for the RNG-function
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 67a7877..35e3976 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -219,7 +219,7 @@
int mbedtls_platform_memcmp( const void *buf1, const void *buf2, size_t num );
/**
- * \brief A global RNG-function
+ * \brief RNG-function for getting a random in given range.
*
* This function is meant to provide a global RNG to be used
* throughout Mbed TLS for hardening the library. It is used
@@ -228,6 +228,12 @@
* cryptographically secure RNG, but provide an RNG for utility
* functions.
*
+ * \note Currently the function is dependent of hardware providing an
+ * rng with MBEDTLS_ENTROPY_HARDWARE_ALT. By default, 0 is
+ * returned.
+ *
+ * \note If the given range is [0, 0), 0 is returned.
+ *
* \param num Max-value for the generated random number, exclusive.
* The generated number will be on range [0, num).
*