Bug 1860451 - Add documentation for the hash inequality in HashPolicy…

…::match. r=mgaudet DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D191591

mfbt/HashTable.h

@@ -732,7 +732,12 @@ class HashSet {
 //  - a static member function |HP::match| that tests equality of key and
 //    lookup values:
 //
-//      static bool match(const Key&, const Lookup&);
+//      static bool match(const Key& aKey, const Lookup& aLookup);
+//
+//    |aKey| and |aLookup| can have different hash numbers, only when a
+//    collision happens with |prepareHash| operation, which is less frequent.
+//    Thus, |HP::match| shouldn't assume the hash equality in the comparison,
+//    even if the hash numbers are almost always same between them.
 //
 // Normally, Lookup = Key. In general, though, different values and types of
 // values can be used to lookup and store. If a Lookup value |l| is not equal