docs($http): explain why $http may not make the request immediately

I came across this issue today and after researching has found out this thread on so:
http://stackoverflow.com/questions/17039998/angular-not-making-http-requests-immediately.

It took me quite sometimes to figure out this so I hope the addition in documentation could save somebody else some times and frustration.
docs($http): add examples when calling $http outside $apply

Closes #3996