net_tstamp: Improve kernel-doc for struct hwtstamp_config

Fix the name of the rx_filter field.

Remove text about 32/64-bit compatibility; this works just the same as
for most socket ioctls and as the structure is not allowed to grow
there is no need to remind anyone how to maintain it.

Add explanation about drivers changing the filter mode.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>