test-lib: add a few comments to LSan log checking

test-lib: add a few comments to LSan log checking

Commit b119a687d4 (test-lib: ignore leaks in the sanitizer's thread
code, 2025-01-01) added code to suppress a false positive in the leak
checker. But if you're just reading the code, the obscure grep call is a
bit of a head-scratcher. Let's add a brief comment explaining what's
going on (and anybody digging further can find this commit or that one
for all the details).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>