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.
It's puzzling me beyond belief. I first saw, of all things, using netcat! I don't use IE... but that showed up there. Now I'm a level-1 rookie, and I know my VPS security settings are very lax (e.g., I use root all the time), so I could only understand it with a benign intruder. Beats me...
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.
Apparently documents IE saves locally are not bit-for-bit faithful to the original. Any chance you downloaded your document and uploaded it again later, maybe after editing locally?
Also, dear Microsoft: PLEASE STOP SMUGGLING METADATA IN COMMENTS! Markup is extensible for a reason. Jeez.
