sanity: add a comment to ensure CONNECTIVITY_CHECK_URIS is correct

In sanity.bbclass, in function check_connectivity :
Connectivity is tested by fetching URIs in variable CONNECTIVITY_CHECK_URIS.
If none is accessible, the status error is ambiguous.

It says to ensure the host's network is correctly configured but
never if the remote is available.

Signed-off-by: Shubham Kulkarni <skulkarni@mvista.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>