bpf, docs: Explain helper functions

Add brief text about existence of helper functions, with details to go in
separate psABI text.

Note that text about runtime functions (kfuncs) is part of a separate patch,
not this one.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>