Comment-only: document sni_hostname more.

Previously we expected the reader to automatically know why it was
called "SNI", which really isn't fair.

crates/tor-rtcompat/src/traits.rs

@@ -183,9 +183,11 @@ pub trait TlsConnector<S> {
 
     /// Start a TLS session over the provided TCP stream `stream`.
     ///
-    /// Declare `sni_hostname` as the desired hostname, but don't
-    /// actually check whether the hostname in the certificate matches
-    /// it.
+    /// Declare `sni_hostname` as the desired hostname, but don't actually check
+    /// whether the hostname in the certificate matches it.  The connector may
+    /// send `sni_hostname` as part of its handshake, if it supports
+    /// [SNI](https://en.wikipedia.org/wiki/Server_Name_Indication) or one of
+    /// the TLS 1.3 equivalents.
     async fn negotiate_unvalidated(&self, stream: S, sni_hostname: &str) -> IoResult<Self::Conn>;
 }