Improve aliases and references doc

[brian-mann]

Indicate improper use of let or const assignment.

Talk about aliasing as this.

Sharing context between hooks + tests.

Call out some of these things as Anti Patterns with link to the doc explaining it all.

[brian-mann]

Fixed by #220.

[iankpconcentricsky]

Usually, replaying previous commands will return what you expect, but not always

Why's that? Can this be elaborated on?

What's the difference between:

cy.get('#nav header .user').as('user') (good)

and

cy.get('#nav').find('header').find('.user').as('user')

it isn't clarified. Just left as a breadcrumb of an unsolved mystery. How would, by grouping the retry, the latter fail where the original succeeds? This seems like it would be an overall best-practice even unrelated to variables?