French translation of notation-appendices.itely breaks make web

[Carl D. Sorensen]

Dear French translators,

The git version of Documentation/fr/user/notation-appendices.itely is
missing a node, which causes make web to fail.

I've added in the node, Common chord modifiers, and it got past that error,
but I don't know if I've done it right.

I've attached a patch, in case you want to use it.  Or I'd be willing to
push it myself if somebody in authority tells me it's OK.

Thanks,

Carl

P.S. I have a question about a style issue.  In notation-appendices.itely,
there are lots of section introductions that begin "Vous pouvez .....".
When I took French many years ago, I was taught that preferred French style
would be "On peut ...".

Is this a change in French style, or was what I learned just wrong?  If both
"Vous pouvez" and "On peut" are acceptable, can you give me some guideline
as to when I would use one or the other?

Thanks,  Carl

french-node.patch
Description: french-node.patch