persist_docs is not called in incremental models

[binhnefits]

Describe the bug

persist_docs adds column comments to delta tables but its not called in incremental models

Steps To Reproduce

Expected behavior

Screenshots and log output

**The operating system you're using: Ubuntu

**The output of python --version: 3.7

Additional context

[jtcohen6]

@binhnefits Thanks for opening!

As I said over in I think it would just look like adding

{% do persist_docs(target_relation, model) %}

to the end of the incremental materialization, right above run_hooks, just like in the table materialization:

dbt-spark/dbt/include/spark/macros/materializations/table.sql

Lines 25 to 27 in f39169e

	{% do persist_docs(target_relation, model) %}
	{{ run_hooks(post_hooks) }}

[jtcohen6]

resolved by #234