verification: Rustdoc comments for Verifier.

This commit adds some missing Rustdoc comments for the various
per-platform `Verifier` structs and associated methods. Doing so will
avoid errors from missing comments once the types are part of the public
API.

src/verification/android.rs

@@ -34,6 +34,7 @@ enum VerifierStatus {
 // official recommendation. See https://bugs.chromium.org/p/chromium/issues/detail?id=627154.
 const AUTH_TYPE: &str = "RSA";
 
+/// A TLS certificate verifier that utilizes the Android platform verifier.
 #[derive(Default)]
 pub struct Verifier {
     /// Testing only: The root CA certificate to trust.
@@ -55,13 +56,16 @@ impl Drop for Verifier {
 }
 
 impl Verifier {
+    /// Creates a new instance of a TLS certificate verifier that utilizes the
+    /// Android certificate facilities
     pub fn new() -> Self {
         Self {
             #[cfg(any(test, feature = "ffi-testing"))]
             test_only_root_ca_override: None,
         }
     }
 
+    /// Creates a test-only TLS certificate verifier which trusts our fake root CA cert.
     #[cfg(any(test, feature = "ffi-testing"))]
     pub(crate) fn new_with_fake_root(root: &[u8]) -> Self {
         Self {

src/verification/apple.rs

@@ -37,6 +37,7 @@ fn system_time_to_cfdate(time: SystemTime) -> Result<CFDate, TlsError> {
         .map(|epoch| CFDate::new(epoch.as_secs() as f64))
 }
 
+/// A TLS certificate verifier that utilizes the Apple platform certificate facilities.
 #[derive(Default)]
 pub struct Verifier {
     /// Testing only: The root CA certificate to trust.

src/verification/others.rs

@@ -5,6 +5,7 @@ use rustls::{
     CertificateError, Error as TlsError, RootCertStore,
 };
 
+/// A TLS certificate verifier that uses the system's root store and WebPKI.
 #[derive(Default)]
 pub struct Verifier {
     // We use a `OnceCell` so we only need

src/verification/windows.rs

@@ -487,6 +487,7 @@ fn map_trust_error_status(unfiltered_status: DWORD) -> Result<(), TlsError> {
     )))
 }
 
+/// A TLS certificate verifier that utilizes the Windows certificate facilities.
 #[derive(Default)]
 pub struct Verifier {
     /// Testing only: The root CA certificate to trust.