doc: fix comment on WebPKI and platform verifier

Since the webpki crate is a specific implementation (which is chosen by the
platform verifier crate on some platforms), it was confusing to call it out by
name. Instead, simplify the comment to say it uses "the default behavior for the
current platform."

src/cipher.rs

@@ -1055,8 +1055,9 @@ box_castable! {
 }
 
 impl rustls_server_cert_verifier {
-    /// Create a new server certificate verifier that uses the system's root store and WebPKI via
-    /// [`rustls-platform-verifier`][].
+    /// Create a verifier that uses the default behavior for the current platform.
+    ///
+    /// This uses [`rustls-platform-verifier`][].
     ///
     /// The verifier can be used in several `rustls_client_config` instances and must be freed by
     /// the application using `rustls_server_cert_verifier_free` when no longer needed.