docs: Surface GWP-ASan probabilities

Change-Id: I4ec4c9c8f5a96ed59e30f3acb388e1ff970dc383
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4751364
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Sergei Glazunov <glazunov@google.com>
Cr-Commit-Position: refs/heads/main@{#1180677}

docs/gwp_asan.md

@@ -97,3 +97,38 @@ GWP-ASan exception.
 
 There is [not yet](https://crbug.com/910749) a way to inspect GWP-ASan data in
 a minidump (crash report) without access to Google's crash service.
+
+## Appendix: Probabilities
+
+The question "shall we enable GWP-ASan at all in this process?" is
+answered by
+
+`base::RandDouble()` < `ProcessSamplingProbability` ×
+`ProcessSamplingBoost2`
+
+where
+
+*   0.0 ≤ `ProcessSamplingProbability` ≤ 1.0,
+
+*   `ProcessSamplingBoost2` ≥ 1, and
+
+*   `base::RandDouble()` has range [0, 1).
+
+The question "on average, how many allocations shall occur before
+GWP-ASan takes a sample?" is answered by
+
+`AllocationSamplingMultiplier` × (`AllocationSamplingRange`
+∗∗ `base::RandDouble()`)
+
+where
+
+*   `AllocationSamplingMultiplier` ≥ 1,
+
+*   `AllocationSamplingRange` ≥ 1, and
+
+*   the final expression is < `max(size_t)`.
+
+As an example, on Linux, using the default parameters and
+`base::RandDouble() == 0.5`, we get
+
+1500 × (16 ∗∗ 0.5) = 6000