bpf, docs: Formalize type notation and function semantics in ISA standard

Give a single place where the shorthand for types are defined and the
semantics of helper functions are described.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230807140651.122484-1-hawkinsw@obs.cr


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