Update Excerpt macro documentation

Issue Summary

Confluence allows multiple excerpt macros to be added to the page, but the excerpt-include macro will only look at the first excerpt macro on the referenced page.

Suggested solution

Our documentation on Excerpt Macros says that:

You can only define one excerpt per page. In other words, you can only add the Excerpt macro once to a page.

We should modify the wording so that it says that although you can define more than one excerpt per page, the excerpt-include macro will only look at the top excerpt macro on the referenced page.