The documentation page for function_output_iterator doesn't answer TQN0

Documentation authors can safely assume that whenever somebody visits a documentation page to see documentation about a particular class, template, function, or any other kind of Thing That Can Be Used In Code, that person **will, with absolute certainty**, need to know the answer to The Question Number Zero (TQN0).

The Question Number Zero goes like this:

> Which header file must I include in order to be able to use this thing in my code?

Please, always answer TQN0 on every documentation page that documents anything that needs an include file in other to be used.

https://www.boost.org/doc/libs/1_78_0/libs/iterator/doc/html/iterator/specialized/function_output.html