BCP 14 boilerplate detection

[martinthomson]

The following warning:

 6  Warning
 └- Code - MISSING_NOTRECOMMENDED_IN_BOILERPLATE
 └- Desc - The keyword NOT RECOMMENDED appears but not included in the RFC2119 boilerplate.
 └- Ref  - https://www.rfc-editor.org/rfc/rfc7322.html#section-4.8.2

...is the only one related to BCP 14. The XML I have looks like so:

      <t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
described in BCP¤14 <xref target="BCP14"/> when, and only when, they
appear in all capitals, as shown here.</t>

(This is the new form that kramdown-rfc produces from {::boilerplate bcp14-tagged-bcp}, which uses a BCP reference rather than the two RFC references that used to be used. I can't see how that leads to this specific problem though.)