Remove unreferenced property `Ember.Comparable#isComparable`

`Ember.Comparable#isComparable` is not referenced from anywhere.
Example for Ember.computed.alias
Merge pull request #3028 from tricknotes/remove-unreferenced-property

Remove unreferenced property `Ember.Comparable#isComparable`
Merge pull request #3025 from trek/computed.alias-example

Example for Ember.computed.alias
Tweak htmlSafe docs
Merge pull request #3035 from mixonic/doc-htmlSafe

Tweak htmlSafe docs