Both XML and HTML forbid double hyphens inside comments. However,
nxml-mode was using a `!--' as a comment padding if `comment-style'
was set to any of the styles that supposed to add padding. This infix
was auto-derived due to `comment-continue' being nil. To fix that set
`comment-continue' explicitly. It's unclear what padding should be
used, but from looking at other editors it seems they don't typically
add padding in XML, so let's be simple for now and just set
`comment-continue' to empty string.
* lisp/nxml/nxml-mode.el (nxml-mode): Make 'comment-continue' a
buffer-local variable set to the empty string. (Bug#71772)