add comment about hashSalt in baseWebpackConfig.js

to make it clear why it's there and under what conditions
it should be changed

refs CORE-933

Change-Id: I449eaaa54baddc3ab6667f2a16d97570bb61f21b
Reviewed-on: https://gerrit.instructure.com/139438
Tested-by: Jenkins
Reviewed-by: Brent Burgoyne <bburgoyne@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>

frontend_build/baseWebpackConfig.js

@@ -83,6 +83,10 @@ module.exports = {
   }, bundleEntries, momentLocaleBundles),
 
   output: {
+    // NOTE: hashSalt was added when HashedModuleIdsPlugin was installed, since
+    // chunkhashes are insensitive to moduleid changes. It should be changed again
+    // if this plugin is reconfigured or removed, or if there is another reason to
+    // prevent previously cached assets from being mixed with those from the new build
     hashSalt: '2018-01-29',
     path: path.join(__dirname, '../public', webpackPublicPath),