comment why this file is needed

debug.go

@@ -2,6 +2,14 @@
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
+// We use a build tag to change the default GODEBUG value to the latest Go
+// release, rather than setting this default in the go.mod for two reasons:
+//
+// 1. Our go.mod specifies at most the previous supported Go version as a
+//    minimum, and the latest Go toolchain's default GODEBUG cannot be specified
+//    without breaking builds with the older toolchains.
+// 2. If a go.work is used, the godebug directives in go.mod will be ignored.
+
 //go:build go1.24
 
 //go:debug default=go1.24