[SPARK-14573][PYSPARK][BUILD] Fix PyDoc Makefile & highlighting issues

## What changes were proposed in this pull request?

The PyDoc Makefile used "=" rather than "?=" for setting env variables so it overwrote the user values. This ignored the environment variables we set for linting allowing warnings through. This PR also fixes the warnings that had been introduced.

## How was this patch tested?

manual local export & make

Author: Holden Karau <holden@us.ibm.com>

Closes #12336 from holdenk/SPARK-14573-fix-pydoc-makefile.