I’ve successfully used the wp-head element to add some hreflang tags to my site (using support forum docs, so thanks for that).
Weirdly, although I can see the tags in the browser source code, I can’t see the comment that I added above them which I borrowed (copy and pasted, so no error in trancription) from your Header Element Overview article:
<!–This is a comment.–>
I then found a comment in the code which was showing and whittled it down until it stopped appearing. Based on that experimentation, comments seem to require the following bits to be viewable in the source code (minus the ‘Your hreflang tags’ bit):