fix(cdk/menu): update docs to reflect current implementation and add correct role for triggers

[mmalerba]

No description provided.

[jelbourn]

LGTM, mostly minor grammar nits

[jelbourn]

lgtm

One thing that occurred to me for a potential follow-up PR is documenting the CSS classes applied by the cdk menu directives

[mmalerba]

I added a section about CSS classes to this PR, but its pretty boring... all of the classes but one were just static always-applied classes. I guess there's a few options for what we could do with the CSS classes:

1. Leave it how it is, pretty much just static classes, e.g. cdk-menu, cdk-menu-trigger, etc
2. Add more dynamic classes, e.g. cdk-menu-opened, cdk-menu-item-selected
3. Just remove all the classes and leave it up to users to add them if they want them (all of the static ones could just be accomplished with an attribute selector anyways, e.g. [cdkMenu])

[jelbourn]

Repeating here from our conversation earlier for posterity:
I think the CSS classes are useful for both inheriting the directives and for when we can do directives on host elements

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