docs(error/$rootScope/inprog): improve understanding and diagnosis of the error

See #5549
docs(guide/databinding): add line breaks after images

This looks much better, thanks a bunch @NV

Closes #7748
docs(CHANGELOG.md): remove mention of strict-DI from v1.2.17

Strict-DI feature was originally merged into v1.2.x, but was reverted by https://github.com/angular/angular.js/commit/373078a94cf3d525b9ae11a2f2876acb6e26f6a3
docs(ngController): improve wording
docs(guide): add ngTagsInput to UI Widgets
docs(guide): add UI-Map to Complementary Libraries
docs(guide/index): add codeschool link
feat(attrs): trigger observers for specific ng-attributes

When an observer is set to listen on the pattern, minlength or maxlength attributes
via $attrs then the observer will also listen on the ngPattern, ngMinlength and the
ngMaxlength attributes as well.

Closes #7758
docs(error/$sce/unsafe): suggest including ngSanatize
feat(ngInclude): emit $includeContentError when HTTP request fails

This adds a scope event notification when a template fails to load.

This can have performance implications, and unfortunately cannot at this moment
be terminated with preventDefault(). But it's nice to be notified when problems
occur!

Closes #5803
docs(tutorial/step_09): note about 'Filter' suffix

Reminder that 'Filter' is appended to filter names when injected.
Link to Filter guide where this is mentioned.
docs(directive/input): document ngTrim for textarea
docs(error/$compile/tplrt): note that html comments can cause this
docs(api/index): add Oxford comma

Verified grammar at https://owl.english.purdue.edu/owl/resource/607/01/
docs($animate): $animate.enabled's param is optional

The docs show that param 'element' (of type DOMElement) is required when it is optional.