Improve documentation

[vadym-k21]

Hi, guys!
Very thankful for this awesome library, but it is very hard to do something very fast if you are not steem expert.
For instance, in docs it is unclear what is:
parentPermlink and parentAuthor for commenting. So at least you can add this link as a reference on usage: https://github.com/steemit/steem-js/blob/master/examples/broadcast.html

[bonustrack]

There is more details about the comment operation (and others) here: https://developers.steem.io/apidefinitions/broadcast-ops-comment