Add comments to explain mocha flag usages.

Author: yifanyang

config/ci.mocha.node.opts

@@ -1,2 +1,8 @@
+# With dot reporter, the full names of test case will not be displayed in log during execution.
+# The name and details of all failed cases will still be appended to log after execution.
+# See https://mochajs.org/#dot-matrix for more information.
 --reporter dot
+
+# Node doesn't exit after mocha test completion. Supplying this flag for force exit.
+# Not applicable in karma + mocha test (browser test).
 --exit

config/mocha.browser.opts

@@ -1,2 +1,6 @@
+# See:
+#   - https://mochajs.org/#usage for more information on usage of mocha flags.
+#   - https://github.com/karma-runner/karma-mocha for more information on all mocha flags which the
+#     karma runner supports.
 --timeout 20000
 --retry 3

config/mocha.node.opts

@@ -1 +1,3 @@
+# Node doesn't exit after mocha test completion. Supplying this flag for force exit.
+# Not applicable in karma + mocha test (browser test).
 --exit