Mention required usage of the type icon in the alert component usage guidelines

Problem

The alert component is being updated in gitlab-org/gitlab-ui#454 (closed) and gitlab-org/gitlab-ui!832 (merged) according to the new specs added in gitlab-org/gitlab-design@7da20685 and gitlab-org/gitlab-design@03d9b104.

This change is an intermediate step for gitlab-org/gitlab-design#626 (closed).

One of the key points being added is the following:

The alert should always contain an icon representing its type

Currently, the docs mention only the optional usage of the dismiss icon but there's no reference for the required usage of the type icon. This could be taken into account when building the Vue component but it could be hard (almost impossible) to restrict usage without the icon in haml templates.

This can lead to future UX and technical debt.

Proposal

To avoid future UX and technical debt, let's stress out the required usage of the type icon in the alert component usage guidelines.