That "injection" was probably done by internet explorer, as stated in the first sentence of that msdn site. I don't have IE, so can't give you a certain answer. Check your site in another browser to see if that motw isn't there. I bet it won't be.
Some security considerations can't be overlooked, granted; but don't fret, it was possibly put there client side(One might even hypothesise that a WYSIWYG editor put it there by default, but I deem you would have seen that). Just give the url to someone and ask them if it appears in the source. If it does, delete it and see if appears again. If it does, then do get paranoid with security, I guess.
Also, dear Microsoft: PLEASE STOP SMUGGLING METADATA IN COMMENTS! Markup is extensible for a reason. Jeez.