docs($log): add the word "safely" to clarify

I was reading the doc and had to read the code to be sure it was safe.  Spelling it out seems easier.

Closes #4760
docs(overview): improve readability

Inserted a couple of line breaks to increase readability.

Closes #4756
docs(guide/understanding-controllers): fix typo in example

Broken - $scope.spicy = 'very';
Works - $scope.spice = 'very';

Closes #4752
docs($http): note that default $http headers can be overridden

Closes #4750
docs(input): fix typo

Closes #4734