docs: networking: timestamping: improve stacked PHC sentence

docs: networking: timestamping: improve stacked PHC sentence

The first paragraph makes no grammatical sense. I suppose a portion of
the intended sentece is missing: "[The challenge with ] stacked PHCs
(...) is that they uncover bugs".

Rephrase, and at the same time simplify the structure of the sentence a
little bit, it is not easy to follow.

Fixes: 94d9f78f4d64 ("docs: networking: timestamping: add section for stacked PHC devices")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://patch.msgid.link/20250512131751.320283-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>