Merge branch ' docs/bpf: Add description of register liveness tracking algorithm'

Eduard Zingerman says:

====================

An overview of the register tracking liveness algorithm.
Previous versions posted here: [1], [2], [3].
- Changes from RFC to v2 (suggested by Andrii Nakryiko):
  - wording corrected to use term "stack slot" instead of "stack spill";
  - parentage chain diagram updated to show nil links for frame #1;
  - added example for non-BPF_DW writes behavior;
  - explanation in "Read marks propagation for cache hits" is reworked.
- Changes from v2 to v3:
  - lot's of grammatical / wording fixes as suggested by David Vernet;
  - "Register parentage chains" section is fixed to reflect what
    happens to r1-r5 when function call is processed (as suggested by David and Alexei);
  - Example in "Liveness marks tracking" section updated to explain
    why partial writes should not lead to REG_LIVE_WRITTEN marks
    (suggested by David);
  - "Read marks propagation for cache hits" section updates:
    - Explanation updated to hint why read marks should be propagated
      before jumping to example (suggested by David);
    - Removed box around B/D in the diagram updated (suggested by Alexei).
- Changes from v3 to v4 (suggested by Edward Cree):
  - register parentage chain diagram updated to explain why r6 mark is
    not propagated;
  - read mark propagation algorithm pseudo-code fixed to correctly
    show "if state->live & REG_LIVE_WRITTEN" stop condition;
  - general wording improvements in section "Liveness marks tracking".

[1] https://lore.kernel.org/bpf/20230124220343.2942203-1-eddyz87@gmail.com/
[2] https://lore.kernel.org/bpf/20230130182400.630997-1-eddyz87@gmail.com/
[3] https://lore.kernel.org/bpf/20230131181118.733845-1-eddyz87@gmail.com/
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>