[Docs] Some minor fixes: Typos, Notations, Formatting

Fixes for:
* Notations: html > HTML; javascript > JavaScript; composer > Composer
* Typos
* Wrong function names: ``escaper`` > ``escape``; ``include`` > ``source`` (in new chapter about ``source``)
* Formatting issues, e.g. remove RST from code blocks, it's not evaluated
* Wording where code sample outputs something: returns > outputs
* ``true`` parameter for ``autoescape`` - not needed any more