docs(ngInit): ng-init is an attribute/class directive, not an element directive

As discussed in comments on https://github.com/angular/angular.js/commit/42ec95ebae716c81087684b55ed8fa8c13888abc#commitcomment-5109829,
ngInit is not an element directive, so @clkao's example should reflect this.

Closes #5879
docs($q): note that function okToGreet is expected to be defined in example

the function okToGreet wasn't defined, so this example wouldn't work properly.

I've decided that instead of adding unrelated code to the example, it should just be noted that the
function is expected to be defined in the lexical scope.

Closes #5878
docs($provide): fix Markdown formatting for provider method

Indentation made Markdown parser think that it’s a block of code.

Closes #5446
docs($http): remove outdated part about $http outside of $apply phase

This removes some outdated advice which no longer is true against the latest angular version.

The information about unit testing with ngMocks remains, because it's always good to have
information like that easily found. This little snippet is not worded perfectly, and is not
a very good example unit test, so additional work is needed here.

Relates to #5206
Closes #5485