uapi/bpf: Add missing description and returns for helper documentation

uapi/bpf: Add missing description and returns for helper documentation

Both description and returns section will become mandatory
for helpers and syscalls in a later commit to generate man pages.

This commit also adds in the documentation that BPF_PROG_RUN is
an alias for BPF_PROG_TEST_RUN for anyone searching for the
syscall in the generated man pages.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220119114442.1452088-1-usama.arif@bytedance.com