Fix documentation for Ember.Select multiple and disabled attributes
Adding comments for 'extend()' method

Additional comments

Added comments for Mixins

Added example using Ember.Mixin

Added some description of create() vs extend()
Test with all feature flags enabled.
Merge pull request #3425 from rjackson/test_with_feature_flags

Test with all feature flags enabled.
Fix typo in link-to docs.
Merge pull request #3338 from davidmcch/master

Adding comment to extend() function
[BUGFIX beta] Added tests failing for issue #3331
[BUGFIX beta] Fire observers for array computed changes.

When dealing with chains of arrays, it is not necessary (or correct) to fire
non-array observers when array contents change, but the array itself does not.

However, for other `ReduceComputedProperty`s, such as `Ember.computed.max`, it
is necessary to fire observers in response to upstream array changes.

Thanks @ssured for pointing this out, and for providing tests!

Fix #3331 #3354.
Merge pull request #3361 from hjdivad/fix-3331

Fire observers for array computed changes.
Merge pull request #3434 from rjackson/fix_typo_in_link_to

Fix typo in link-to docs.
Merge pull request #3402 from quadule/fix-select-docs

Fix documentation for Ember.Select multiple and disabled attributes