Clarify the use of MBEDTLS_ERR_PK_SIG_LEN_MISMATCH
Clarify what MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH and
MBEDTLS_ERR_PK_SIG_LEN_MISMATCH mean. Add comments to highlight that
this indicates that a valid signature is present, unlike other error
codes. See
https://github.com/ARMmbed/mbedtls/pull/1149#discussion_r178130705
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index b00ba4d..7b8ffff 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -36,7 +36,7 @@
#define MBEDTLS_ERR_ECP_ALLOC_FAILED -0x4D80 /**< Memory allocation failed. */
#define MBEDTLS_ERR_ECP_RANDOM_FAILED -0x4D00 /**< Generation of random value, such as (ephemeral) key, failed. */
#define MBEDTLS_ERR_ECP_INVALID_KEY -0x4C80 /**< Invalid private or public key. */
-#define MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH -0x4C00 /**< Signature is valid but shorter than the user-supplied length. */
+#define MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH -0x4C00 /**< The buffer contains a valid signature followed by more data. */
#define MBEDTLS_ERR_ECP_HW_ACCEL_FAILED -0x4B80 /**< ECP hardware accelerator failed. */
#if !defined(MBEDTLS_ECP_ALT)