[docs] Improve MediaChange documentation

[aciccarello]

Bug, feature request, or proposal:

Docs request

What is the expected behavior?

MediaChange properties should be JSDoc annotated. Additionally, the wiki page should clearly explain how to use the MediaChange interface.

What is the current behavior?

MediaChange properties have no JSDoc documentation. The ObservableMedia wiki page does not describe any of the properties of MediaChange.

What are the steps to reproduce?

View MediaChange src and ObservableMedia wiki.

What is the use-case or motivation for changing an existing behavior?

Developer ease of use/clarity

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?

My understanding is that the changes object doesn't actually contain helpful information other than mqAlias and usually media.isActive() should usually be used for logic checks.

[CaerusKaru]

Part of #46 and #569, but we'll make sure to take care of this specifically, too.

[adamdport]

The docs should call out explicitly that mqAlias can only be sm, lg, etc, whereas media.isActive() can use gt-sm, lt-lg, etc. I got hung up trying to use the former and thought I needed change.mqAlias === 'md' || change.mqAlias === 'lg' || change.mqAlias === 'xl'

[CaerusKaru]

Added it to the Wiki here. Will add JSDoc in a follow-up PR.

[CaerusKaru]

JSDoc PR: #920

[micobarac]

Documentatation seems to be incorrect.

It states that:

mqAlias - e.g. gt-sm, md, gt-lg, defaults to ''

But:

mqAlias doesn't work for gt-sm, gt-md, ...!!!

[CaerusKaru]

It will in the next release, which should be out within the next couple weeks. If you use the nightly builds, the fix is already committed.

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