Add documentation for block + unblock endpoints

[fitztrev]

https://github.com/lichess-org/lila/blob/f48a2f13dd61a9ec5c8e6ca967fb6e4c6eb2f65e/conf/routes#L51-L52

See existing docs for follow/unfollow for example:

api/doc/specs/lichess-api.yaml

Lines 539 to 543 in 658e06f

	/api/rel/follow/{username}:
	$ref: './tags/relations/api-rel-follow-username.yaml'
	/api/rel/unfollow/{username}:
	$ref: './tags/relations/api-rel-unfollow-username.yaml'

[jalpp]

Hey, I can take care of this please assign to me

[fitztrev]

@jalpp would you still like this?

[jalpp]

Hey! Yes sorry been busy for something else I would get to it soon