Add attributes.derive.behavior

This adds a basic rule for the behavior of derive.

I think there is more to be said here, this is just a starting point.
See things like rust-lang#540 and
rust-lang#1074.

Author: ehuss

src/attributes/derive.md

@@ -51,6 +51,8 @@ Built-in derives are defined in the [language prelude][names.preludes.lang]. The
 - [`PartialEq`]
 - [`PartialOrd`]
 
+r[attributes.derive.behavior]
+During macro expansion, for each element in the list of derives, the corresponding derive macro expands to zero or more [items].
 
 r[attributes.derive.automatically_derived]
 ## The `automatically_derived` attribute