Clarify include / exclude precedence

[robw-mercury]

While reading the docs, I found a sentence that was ambiguous and potentially misleading.

Note: The boundaries/include option has preference over boundaries/ignore. If you define boundaries/include, use boundaries/ignore to ignore subsets of included files.

"boundaries/include has preference" could mean two things:

1. boundaries/include is applied first, and boundaries/ignore is applied after. boundaries/exclude has precedence.
2. boundaries/include is preferred. boundaries/include has precedence.

I edited the docs to use the word "precedence", which removes the ambiguousness.