Commenting like this needs more effort(Drawing arrow etc).
How about giving options like this??
I think that these ideas are preferable:
http://forums.ni.com/t5/LabVIEW-Idea-Exchange/pop-up-comment-on-Block-Digram/idi-p/998374
http://forums.ni.com/t5/LabVIEW-Idea-Exchange/Description-and-Tip-like-Help-Hint-Comments-for-Block-...
And this, which is pretty close to your idea:
http://forums.ni.com/t5/LabVIEW-Idea-Exchange/Ability-to-link-a-free-text-label-or-comment-box-to-a-...
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