docs: netlink: clarify the historical baggage of Netlink flags

nlmsg_flags are full of historical baggage, inconsistencies and
strangeness. Try to document it more thoroughly. Explain the meaning
of the ECHO flag (and while at it clarify the comment in the uAPI).
Handwave a little about the NEW request flags and how they make
sense on the surface but cater to really old paradigm before commands
were a thing.

I will add more notes on how to make use of ECHO and discouragement
for reuse of flags to the kernel-side documentation.

Link: https://lore.kernel.org/r/20220927212306.823862-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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