Clarify output_length in mbedtls_gcm_update()

Signed-off-by: Chien Wong <m@xv97.com>
diff --git a/tf-psa-crypto/drivers/builtin/include/mbedtls/gcm.h b/tf-psa-crypto/drivers/builtin/include/mbedtls/gcm.h
index c27f2dc..764b7c9 100644
--- a/tf-psa-crypto/drivers/builtin/include/mbedtls/gcm.h
+++ b/tf-psa-crypto/drivers/builtin/include/mbedtls/gcm.h
@@ -278,7 +278,8 @@
  *                      of at least \p output_size bytes.
  * \param output_size   The size of the output buffer in bytes.
  * \param output_length On success, \p *output_length contains the actual
- *                      length of the output written in \p output.
+ *                      length of the output written in \p output, which equals
+ *                      to \p input_length.
  *                      On failure, the content of \p *output_length is
  *                      unspecified.
  *