Improve the documentation of OpamSwitchState.dependencies

src/state/opamSwitchState.mli

@@ -167,11 +167,14 @@ val depexts: 'a switch_state -> package -> OpamSysPkg.Set.t
 (** Return the transitive dependency closures
     of a collection of packages.
 
-    [depopts]: include optional dependencies (depopts: foo)
-    [build]: include build dependencies (depends: foo {build})
-    [post]: include post dependencies (depends: foo {post})
-    [installed]: only consider already-installed packages
-    [unavaiable]: also consider unavailable packages
+    @param depopts include optional dependencies (depopts: foo)
+    @param build include build dependencies (depends: foo {build})
+    @param post include post dependencies (depends: foo {post})
+    @param installed only consider already-installed packages
+    @param unavaiable also consider unavailable packages.
+                      If the availability of packages hasn't been computed yet,
+                      setting this [false] can have a significant performance
+                      impact depending on the platform.
 *)
 val dependencies:
   'a switch_state -> build:bool -> post:bool -> depopts:bool ->