runtime/debug: document DefaultGODEBUG as a BuildSetting

Fixes #66465

Change-Id: I60c017ddba29fa5b452b665d8521cd6c8e20438c
Reviewed-on: https://go-review.googlesource.com/c/go/+/597979
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: seankhliao

src/runtime/debug/mod.go

@@ -77,6 +77,7 @@ type Module struct {
 //   - CGO_CPPFLAGS: the effective CGO_CPPFLAGS environment variable
 //   - CGO_CXXFLAGS:  the effective CGO_CXXFLAGS environment variable
 //   - CGO_LDFLAGS: the effective CGO_LDFLAGS environment variable
+//   - DefaultGODEBUG: the effective GODEBUG settings
 //   - GOARCH: the architecture target
 //   - GOAMD64/GOARM/GO386/etc: the architecture feature level for GOARCH
 //   - GOOS: the operating system target