Group - 'iterate' documentation and guard

Update iterate documentation to cover usage of `args` and added a guard so
that the callback can be used without requiring that `args` is specified.

Ref. https://github.com/photonstorm/phaser/issues/1352
Group - additional documentation refinement

General tidying/corrections of Group documentation.
Documentation - general

Updated some documentation for formatting, consistency, and minor
corrections.
Documentation templates - edited resource entries

Added Game and Particle entries.
Documentation - consistency updates

- Updated `readOnly` doclet to `readonly`

- `array` refined to `type[]`, where such information was immediately
determinable.

- Updated {Any}/{*} to {any}; {...*} is standard exception

- Udated {Object} to {object}
Documentation - typo fix
General docs - reverting src/pixi changes

(Because outside of control, really.)
Documentation - Group

- Merged in group doc updates

- Added a DisplayObject stub type for better link-out across documentation
Documentation - number refinement

Changed some {number} types to {integer} where such is trivially
verifiable.
Documentation - Signal
Merge pull request #1386 from pnstickne/wip-docs-1130

Assorted documentation/consistency updates