internal/poll: document why zero-byte reads do not wait for readability

[fyzanshaik]

The TODO asked whether a zero-byte Read should wait for readability.
CL 22031 proposed that behavior and was abandoned. Blocking would
change the behavior of existing callers, and the netpoller's
edge-triggered notifications alone cannot detect data that is
already buffered. Replace the stale TODO with the outcome.

Updates #15735

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Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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