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

[holdenk]

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

[holdenk]

cc @BryanCutler who ran into this issue

[SparkQA]

Test build #55648 has finished for PR 12336 at commit 3f19309.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55652 has finished for PR 12336 at commit 3b85ed0.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55669 has finished for PR 12336 at commit d4f9687.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[BryanCutler]

?= seems to do the trick, LGTM

[srowen]

LGTM

[holdenk]

If this looks OK to people it would be good to merge before anymore PyDoc highlighting inconsistencies make it into master. Maybe @JoshRosen or @davies could merge could also take a quick look and merge this if its ok as they are the most recent committers on the Makefile?

[srowen]

Looks like a clean improvement; merged to master