encoding/json/v2: document context annotation with SemanticError
When the json package calls
Marshaler, MarshalerTo, Unmarshaler, or UnmarshalerFrom methods
and a SemanticError is returned, it will automatically
annotate the error with context.
Document this behavior.
Change-Id: Id8e775a7c1c2a6ffc29ea518913591915e8aff87
Reviewed-on: https://go-review.googlesource.com/c/go/+/701956 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>