PHPDoc improvements for SiteTree and SiteTreeLinkTracking

- Removed @param/@method references to fields from extensions
- Added @mixin references to extensions (supported by PHPStorm, should also be by Netbeans)
- Fixed incorrect PHPDoc @params and @returns
- Fixed wrapping width on many docblocks
- Added and/or improved method documentation when missing or confusing
- General tweaks and fixes to match latest standards and IDE capabilities
- Fixed typos as found

For the Framework counterpart see https://github.com/silverstripe/silverstripe-framework/pull/3698