docs/bpf: Document arena pointers in a by-value return

Section 2.9 describes the by-value return contract as scalars only, which
no longer holds: a kfunc and a global subprogram may now return a struct
or union whose members are scalars or arena pointers. Update it.

Note that an arena pointer member is handed back as a scalar like every
other member. That costs nothing: the program has to cast_kern() the value
before it can be used, and the verifier allows that cast on any scalar, so
the member gives the program no reach it did not already have.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061615.1700421-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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