improve documentation

lib/ca_certs.mli

@@ -1,11 +1,16 @@
-val trust_anchors : unit -> (string, [> `Msg of string ]) result
-(** [trust_anchors ()] returns the trust anchors of the host system, if
-    found. *)
-
 val authenticator :
   ?crls:X509.CRL.t list ->
   ?hash_whitelist:Mirage_crypto.Hash.hash list ->
   unit ->
   (X509.Authenticator.t, [> `Msg of string ]) result
-(** Detects root CAs in the operating system's trust store.
+(** [authenticator ~crls ~hash_whitelist ()] detects the root CAs (trust
+    anchors) in the operating system's trust store using {!trust_anchors}. It
+    constructs an authenticator with the current timestamp {!Ptime_clock.now},
+    and the provided [~crls] and [~hash_whitelist] arguments, to be used for
+    {!Tls.Config.client}.
     Returns [Error `Msg msg] if detection did not succeed. *)
+
+val trust_anchors : unit -> (string, [> `Msg of string ]) result
+(** [trust_anchors ()] detects the root CAs (trust anchors) in the operating
+    system's trust store.
+    The successful result is a list of pem-encoded X509 certificates. *)