docs: qualify what rule 2's baseline number depends on, audited rather than assumed

Both queues are empty, so this turn audited a hole the gate cannot see by
construction: a DISABLED_ test is omitted from gtest's run total AND never
appears as skipped, so it could hide from the count and from the "0 skipped"
claim simultaneously.

Measured: ZERO DISABLED_ tests in the repository. Clean.

Measured: 56 GTEST_SKIP sites, none of which fires here, all environment-
conditional -- chiefly tzdata zone availability (~21), /proc descriptor counts
(8), signal delivery (4), locale availability (3) and DNS (2). Two gtest_filter
strings exist in source, both in PosixSignalTests' re-exec child invocation,
which is #1979's design rather than an exclusion.

That leads to a property of the baseline that rule 2 did not record, and it was
PROVED with a three-test probe rather than reasoned: a conditional GTEST_SKIP
leaves "N tests ... ran" unchanged and drops PASSED by one. So the run total is
environment-independent with respect to those 56 skips -- a container without
America/New_York tzdata still reads run=17,585, with ~21 skipped. The floor this
rule protects is intact there, which is what a no-regression rule needs, and
which the existing qualification about FAILURES differing by environment does
not by itself say.

Docs only; no code changed, gate unaffected at 17,585.

Author: robertvokac