Add documentation for set_root_ca_cert overload

Author: facchinm

connectivity/netsocket/include/netsocket/TLSSocketWrapper.h

@@ -102,6 +102,9 @@ class TLSSocketWrapper : public Socket {
      * @retval NSAPI_ERROR_OK on success.
      * @retval NSAPI_ERROR_NO_MEMORY in case there is not enough memory to allocate certificate.
      * @retval NSAPI_ERROR_PARAMETER in case the provided root_ca parameter failed parsing.
+     *
+     * @note The user can also provide root_ca as a filesystem path (containing files in any Mbed TLS-supported format)
+     * if len == 0 and MBEDTLS_FS_IO is defined
      */
     nsapi_error_t set_root_ca_cert(const void *root_ca, size_t len);