Add comment about matched var.

Author: dlongley

lib/ProofSet.js

@@ -320,7 +320,10 @@ async function _matchProofSet({
       continue;
     }
 
-    // next, find the suite that can verify the proof
+    // next, find the suite that can verify the proof; if found, `matched`
+    // will be set to `true` and the proof will be added to `purposeToProofs`
+    // and `proofToSuite` to be processed -- otherwise it will not be; if
+    // no proofs are added for a given purpose, an exception will be thrown
     let matched = false;
     for(const s of suites) {
       // `matchingProofs` is a map of promises that resolve to whether a