Fix platform setup/teardown feature and comments

Fixed the platform setup/teardown feature, by fixing it for doxygen and adding it
as a feature  in 'version_features.c'.

include/mbedtls/platform.h

@@ -326,8 +326,6 @@ int mbedtls_platform_setup( mbedtls_platform_context *ctx );
  *
  * \param   ctx     mbed TLS context
  *
- * \return  0 if successful
- *
  * \note    This function should be called after every other mbed TLS module has
  *          been correctly freed using the appropriate free function.
  *          Its implementation is platform specific, and by default, unless

library/version_features.c

@@ -69,6 +69,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_PLATFORM_NV_SEED_ALT)
     "MBEDTLS_PLATFORM_NV_SEED_ALT",
 #endif /* MBEDTLS_PLATFORM_NV_SEED_ALT */
+#if defined(MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT)
+    "MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT",
+#endif /* MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT */
 #if defined(MBEDTLS_DEPRECATED_WARNING)
     "MBEDTLS_DEPRECATED_WARNING",
 #endif /* MBEDTLS_DEPRECATED_WARNING */