This commit makes a number of changes to the docs for Labels and
annotations.
1. It changes the order in which Labels and annotations are mentioned
from Annotations and labels to Labels and annotations as this is
the order shown in the UI when creating and editing alert rules.
It is also the order in the Prometheus documentation.
2. It changes most of the documentation explaining what labels and
annotations are and the differences between them. It also adds
some paragraphs on Custom Labels.