code: public final LoggingAdvisingAppendable setSanitizedContentKind(ContentKind kind)
        throws IOException {
      checkNotNull(kind);
      if (this.kind == null) {
        this.kind = kind;
        notifyContentKind(kind);
      }
      return this;
    }
docs: Marks the content kind of this appendable. All the {@link #append} commands should be
  considered to be content of the given kind that has already been sanitized.
  
  <p>Calls after the first call to this are ignored. In the case of an appendable's content kind
  changing, we trust that the autoescaper has inserted the correct escape directives, so we do
  not need to track this in the appendable.