docs/bpf: Add description of register liveness tracking algorithm

This is a followup for [1], adds an overview for the register liveness
tracking, covers the following points:
- why register liveness tracking is useful;
- how register parentage chains are constructed;
- how liveness marks are applied using the parentage chains.

[1] https://lore.kernel.org/bpf/CAADnVQKs2i1iuZ5SUGuJtxWVfGYR9kDgYKhq3rNV+kBLQCu7rA@mail.gmail.com/

Signed-off-by: Eduard Zingerman <[email protected]>
Reviewed-by: Edward Cree <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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