doc: Add link to J-Link virtual MSD disable for SMP

Virtual MSD J-Link support on some development boards has caused an
issue with SMP due to limiting the maximum size of UART data via the CDC
endpoint, add a link to the SMP documentation and smp_svr sample
application on how to disable MSD functionality and resolve the issue.

Signed-off-by: Jamie McCrae <jamie.mccrae@lairdconnect.com>