Doc support for JAVA_HOME
Doc support for JAVA_HOME

Summary:
- Re-emphasize that Buck requires Java 8.

- Be nondescript about which future version we'll support.

- Explain how to set JAVA_HOME to the correct directory if the
  development computer has multiple versions of Java installed.

- Clarify the names of the Java compiler and runtime and that these have `exe` extensions on Windows.

Reviewed By: bobyangyf

fbshipit-source-id: 2cdea7f

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