Improves readability but rearranging the list of refable nodes for expected property in exposures (#5576)

## What are you changing in this pull request and why?
This PR reorders the list of refable node options for `Expected`
property to improve readability:

* Include the comment about the source immediately after the source
node. This comes after the metric node now, which can be an additional
mental jump.

* Downstream to upstream: this is subtle, but it would help to first
list metric, then ref, and finally source nodes to communicate likely
downstream node exposures before unlikely upstream node exposures.

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Author: mirnawong1

website/docs/docs/build/exposures.md

@@ -84,7 +84,7 @@ _Required:_
 <VersionBlock firstVersion="1.4">
 
 _Expected:_
-- **depends_on**: list of refable nodes, including `ref`, `source`, and `metric` (While possible, it is highly unlikely you will ever need an `exposure` to depend on a `source` directly)
+- **depends_on**: list of refable nodes, including `metric`, `ref`, and `source`. While possible, it is highly unlikely you will ever need an `exposure` to depend on a `source` directly.
 
 </VersionBlock>