Add optional/required markers to the flag descriptions

[madhusudancs]

@perotinus I like this change!

Please rebase?

[nikhiljindal]

optional: I was thinking of adding them to the start of the description. That way its easier to parse which flags are important and which are not (descriptions are displayed left aligned).

[perotinus]

@nikhiljindal Done. @madhusudancs Done.