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 <[e-mail]>
Acked-by: Richard Cochran <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/timestamping.rst

···
 3.2.4 Other caveats for MAC drivers
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Stacked PHCs, especially DSA (but not only) - since that doesn't require any
-modification to MAC drivers, so it is more difficult to ensure correctness of
-all possible code paths - is that they uncover bugs which were impossible to
-trigger before the existence of stacked PTP clocks.  One example has to do with
-this line of code, already presented earlier::
+The use of stacked PHCs may uncover MAC driver bugs which were impossible to
+trigger without them. One example has to do with this line of code, already
+presented earlier::
 
       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;