Add note about decorator loading in Engines guide.

Because decorators aren't referenced by the application, they won't get
autoloaded. And because we recommend the pattern, it would be irresponsible
to not show how to load them properly.

Fixes #10647.
Fix some typo in method names, variables
compatability => compatibility
Merge pull request #10761 from vipulnsward/fix-typo-1

Fix some typo in method names, variables
Merge pull request #10762 from vipulnsward/change_compatibility

compatability => compatibility