Added some documentation to Karma configuration files.

common/lib/xmodule/xmodule/js/karma_xmodule.conf.js

@@ -1,5 +1,5 @@
 // Karma config for xmodule suite.
-// Docs in common/static/common/js/karma.common.conf.js
+// Docs and troubleshooting tips in common/static/common/js/karma.common.conf.js
 
 /* jshint node: true */
 /*jshint -W079 */
@@ -16,6 +16,7 @@ var options = {
         return pattern;
     },
 
+    // Avoid adding files to this list. Use RequireJS.
     libraryFilesToInclude: [
         {pattern: 'common_static/js/vendor/requirejs/require.js', included: true},
         {pattern: 'RequireJS-namespace-undefine.js', included: true},
@@ -55,6 +56,8 @@ var options = {
         {pattern: 'common_static/edx-pattern-library/js/**/*.js'}
     ],
 
+    // Make sure the patterns in sourceFiles and specFiles do not match the same file.
+    // Otherwise Istanbul which is used for coverage tracking will cause tests to not run.
     sourceFiles: [
         {pattern: 'src/xmodule.js', included: true, ignoreCoverage: true}, // To prevent getting instrumented twice.
         {pattern: 'src/**/*.js', included: true}