Separate Go discovery configuration from diagnostics

docs/internal/gotool.md

@@ -20,6 +20,10 @@ and native cgo/C-tool configuration into its settings. This captures values
 supplied by either the process environment or the persistent Go environment
 file.
 
+Configuration discovery parses JSON from stdout. Stderr is captured separately
+and included in command-failure diagnostics, so a successful query can emit
+warnings while supplying valid configuration.
+
 ## Command environments
 
 Every Go command receives `GOENV=off`, so removed persistent values cannot