Document incompatibility of VerifyNone with t.Parallel (#107)

It's a known issue that goleak is incompatible with tests being run in
parallel.
* #16
* #83

Unfortunately, there is no real solution to this issue.
* #16 (comment)
* #83 (comment)
* #83 (comment)

This PR at least documents the incompatibility
and suggests using `VerifyTestMain` instead.

leaks.go

@@ -85,6 +85,12 @@ type testHelper interface {
 // tests by doing:
 //
 //	defer VerifyNone(t)
+//
+// VerifyNone is currently incompatible with t.Parallel because it cannot
+// associate specific goroutines with specific tests. Thus, non-leaking
+// goroutines from other tests running in parallel could fail this check.
+// If you need to run tests in parallel, use [VerifyTestMain] instead,
+// which will verify that no leaking goroutines exist after ALL tests finish.
 func VerifyNone(t TestingT, options ...Option) {
 	opts := buildOpts(options...)
 	var cleanup func(int)