Update comment re: private method rename: _add_ctes

[jtcohen6]

Describe the bug

In #2834, it looks like we switched the name of the internal method from _insert_ctes to _add_ctes. That's fine, we should just also update the comment that serves as a hint for plugin authors:

https://github.com/fishtown-analytics/dbt/blob/3af02020ff697cbc34b7bf7d538e99ccd1979afd/core/dbt/compilation.py#L353-L358

Context

Adapter plugin authors for databases that don't support nested CTEs should have a clear point of intervention to alter dbt's behavior when constructing data tests and ephemeral models:

• dbt-synapse#25
• dbt-sqlserver#26

If we have a little extra time, I'd like to prove it's actually possible to override this behavior for data tests and ephemeral models by overriding _add_ctes in an adapter plugin.