No documentation for join() for Custom Adapters

[gaurav21r]

Porting from balderdashy/waterline#1131 ,

I was wondering how adapters can override populate() in my adapter while going through balderdashy/waterline#1052

There seems to be No documentation on how to do this either here-> https://github.com/balderdashy/sails-docs/blob/0.9/adapter-specification.md
Which seems to be the major place for documentation of adapters.

I found it manually while looking over sails-mongo and sails-mysql. The fact that the method's name is join() while accessing it outside the adapter is populate() is counter intuitive. At least there should be docs for this.

@particlebanana @dmarcelino @devinivy What do you say?