Edited 404 documentation for #81
Edited 404 documentation for #81

- Incorporated the feedback I presented.
- Modified the structure to present the two possible ways to do 404
  handling (either explicitly delegating to error middleware and/or the
  final handler, or generating and returning a 404 response).

acelaya/zend-expressive,moderndeveloperllc/zend-expressive,ezimuel/zend-expressive,weierophinney/zf-docs-toolchain-experiment,zf2timo/zend-expressive,nightlinus/zend-expressive