document "brackets" attribute on <eref>

[reschke]

See https://pypi.org/project/xml2rfc/2.34.0/:

Added support for a new attribute ‘brackets { “none” | “angle” }? for , on request from the RPC.

Also:

• https://trac.tools.ietf.org/tools/xml2rfc/trac/changeset/3409/trunk/cli/xml2rfc/data/v3.rnc
• https://greenbytes.de/tech/webdav/rfc2629xslt/rfc2629xslt.html#ext-rfc2629.eref
• reschke/xml2rfc@f3c3145