docs: ethtool: clarify the bit-by-bit bitset format description

docs: ethtool: clarify the bit-by-bit bitset format description

Clarify the bit-by-bit bitset format's behavior around mandatory
attributes and bit identification. More specifically, the following
changes are made:

* Rephrase a misleading sentence which implies name and index are
  mutually exclusive
* Describe that ETHTOOL_A_BITSET_BITS nest is mandatory
* Describe that a request fails if inconsistent identifiers are given

Signed-off-by: Yohei Kojima <yk@y-koj.net>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/ef90a56965ca66e57aa177929ce3e10c5ca815fa.1772031974.git.yk@y-koj.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>