## What changes were proposed in this pull request?

`FsHistoryProviderSuite` fails if `root` user runs it. The test case **SPARK-3697: ignore directories that cannot be read** depends on `setReadable(false, false)` to make test data files and expects the number of accessible files is 1. But, `root` can access all files, so it returns 2.

This PR adds the assumption explicitly on doc. `building-spark.md`.

## How was this patch tested?

This is a documentation change.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15291 from dongjoon-hyun/SPARK-17412.