Documentation/bpf: Document up to 16-byte kfunc return values in R0: R2

kfuncs may now return a value larger than 8 bytes and up to 16 bytes (a
scalar-only struct or union, or an __int128), passed back in the R0:R2
register pair. Add a kfunc return-value section documenting this,
including that a struct or union up to 8 bytes is returned in R0 alone,
which struct and union members are accepted, that the R0:R2 register pair
requires JIT support (bpf_jit_supports_kfunc_ret_reg_pair()), and that a
return value larger than 16 bytes is unsupported.

Also note that the same convention applies to BPF subprogram returns, and
document the consequence for a global subprogram: it must assign both
halves of a register-pair return, since an unassigned R2 may be left
holding a pointer argument and is then rejected as a leak.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://patch.msgid.link/20260819055330.3301120-1-yonghong.song@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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