refactor(netxlite/errors): improve docs and format code (#481)

No real functional change. A few are needed and they will come
next. With this diff I just wanted to do cosmetic changes and
documentation changes, to ensure this package is okay.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/errorsx/operations.go

@@ -1,6 +1,7 @@
 package errorsx
 
-// Operations that we measure.
+// Operations that we measure. They are the possibly values of
+// the ErrWrapper.Operation field.
 const (
 	// ResolveOperation is the operation where we resolve a domain name.
 	ResolveOperation = "resolve"