Improve online docs for `includes:` field

Bases:
* https://downloads.haskell.org/~ghc/6.10.1/docs/html/users_guide/release-6-10-1.html
* https://hackage.haskell.org/package/Cabal-2.0.0.2/changelog
* https://github.com/haskell/cabal/commit/e1c39fc9ff4bf4eb8e956e2ee77da95d6cc73bbe

doc/cabal-package-description-file.rst

@@ -1954,6 +1954,11 @@ system-dependent values for these fields.
    Like :pkg-field:`ghc-prof-shared-options` but applies to GHCJS
 
 .. pkg-field:: includes: filename list
+    :since: 1.0
+    :deprecated: 2.0
+
+    From GHC 6.10.1, :pkg-field:`includes` has no effect when compiling with
+    GHC. From Cabal 2.0, support for GHC versions before GHC 6.12 was removed.
 
     A list of header files to be included in any compilations via C.
     This field applies to both header files that are already installed