bpf/docs: Document how nested trusted fields may be defined

A prior change defined a new BTF_TYPE_SAFE_NESTED macro in the verifier
which allows developers to specify when a pointee field in a struct type
should inherit its parent pointer's trusted status. This patch updates
the kfuncs documentation to specify this macro and how it can be used.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230125143816.721952-7-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>