bpf, docs: Fix small typo and define semantics of sign extension

Add additional precision on the semantics of the sign extension
operations in BPF. In addition, fix a very minor typo.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230808212503.197834-1-hawkinsw@obs.cr


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>