Document no-op pass-through attributes

[pnkfelix]

For example, license, and there are probably a bunch more.

We need to list them explicitly because the manual currently says that all attribute names are currently reserved, so if someone is going to use attributes like license, we need to provide an assurance that rustc will not start interpreting it.

Sub-bug of #6444.

[catamorphism]

Nominating (it sounds sort of like a backwards-compatibility issue).

[catamorphism]

De-nominating since we'll get to the meta-bug.

[steveklabnik]

After pinging @pnkfelix this morning, we're not sure there are any more of these, and since I'm knocking out the rest of #6444, anything missing will get covered there.