fix(11-08): document foreign and nested transaction refusal

- lagoon.Transaction doc and README state that a nested call over a root
  handle returns an error instead of opening an independent transaction
- beachcomber README no longer promises an immediate sync inside a plain
  GORM transaction; it is warned and skipped since 11-08

modules/lagoon/transaction.go

@@ -42,6 +42,9 @@ func (b *afterCommitBuffer) take() []func(context.Context, *gorm.DB) {
 // use the ctx and tx it receives. A Transaction nested in another becomes a
 // savepoint: its callbacks join the outer transaction's only when fn
 // succeeds, so work dropped with the savepoint never runs its callbacks.
+// A nested Transaction must receive the outer transaction's handle; given
+// a root handle it returns an error without running fn, rather than open
+// an independent transaction whose callbacks would wait on the outer one.
 // A panicking callback is logged and never turns a committed write into an
 // error.
 func Transaction(ctx context.Context, gdb *gorm.DB, fn func(ctx context.Context, tx *gorm.DB) error) error {