Clarify docs for MBEDTLS_HAVE_TIME_DATE
/**
* \def MBEDTLS_HAVE_TIME_DATE
*
- * System has time.h, time(), an implementation for mbedtls_platform_gmtime(),
- * and the clock is correct.
+ * System has time.h, time(), an implementation for mbedtls_platform_gmtime()
+ * (see below), and the clock is correct.
* The time needs to be correct (not necesarily very accurate, but at least
* the date should be correct). This is used to verify the validity period of
* X.509 certificates.
