Explain what happens to the old keys in olm

lib/crypto/index.js

@@ -172,6 +172,9 @@ Crypto.prototype.uploadKeys = function(maxKeys) {
         // Try to keep at most half that number on the server. This leaves the
         // rest of the slots free to hold keys that have been cliamed from the
         // server but we haven't recevied a message for.
+        // If we run out of slots when generating new keys then olm will
+        // discard the oldest private keys first. This will eventually clean
+        // out stale private keys that won't receive a message.
         var keyLimit = Math.floor(maxOneTimeKeys / 2);
         // We work out how many new keys we need to create to top up the server
         // If there are too many keys on the server then we don't need to