Add MSS clamp example

[SquirePug]

Adding a complete example on out to apply the Policy Route config directive to adjust MSS, given the require co-configuration directives 'protocol tcp' and 'tcp flags SYN', which are missing from the brief description of the mss adjust feature in the policy route page.

[rebortg]

@SquirePug thx you very much for the example. Just one small think. please use a private IP for your example not the NN.NN.NN..... This will make it more clear.