## What changes were proposed in this pull request?

Modified the documentation to clarify that `build/mvn` and `pom.xml` always add Java 7-specific parameters to `MAVEN_OPTS`, and that developers can safely ignore warnings about `-XX:MaxPermSize` that may result from compiling or running tests with Java 8.

## How was this patch tested?

Rebuilt HTML documentation, made sure that building-spark.html displays correctly in a browser.

Author: frreiss <frreiss@us.ibm.com>

Closes #15005 from frreiss/fred-17421a.