Update incremental-models.md (#6739)

update conditions for is_incremental() to evaluate to true based on
Issue 5279

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This update is to address [Issue 5279 - Understand the is_incremental()
macro](#5279)

I believe the original intent of the first two conditions was to say:
"the model has to exist as a table" however saying this using two lines
feels redundant and I know from training sessions that users react much
the same way as the author of this issue.

I'm proposing the following wording:

Revised

The `is_incremental()` macro powers incremental materializations. It
will return `True` if _all_ of the following conditions are met:

- The model must already exist _as a table_ in the database
- The `full-refresh` flag _is not_ passed
- The running model is configured with `materialized='incremental'`

Original (for reference)

The `is_incremental()` macro powers incremental materializations. It
will return `True` if _all_ of the following conditions are met:

- The model must already exist in the database
- The destination table already exists in the database
- The `full-refresh` flag _is not_ passed
- The running model is configured with `materialized='incremental'`

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https://docs-getdbt-com-git-datapackdan-patch-issue-5279-dbt-labs.vercel.app/docs/build/incremental-models

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website/docs/docs/build/incremental-models.md

@@ -34,8 +34,7 @@ To use incremental models, you also need to tell dbt:
 
 The `is_incremental()` macro powers incremental materializations. It will return `True` if _all_ of the following conditions are met:
 
-- The model must already exist in the database
-- The destination table already exists in the database
+- The model must already exist as a table in the database
 - The `full-refresh` flag _is not_ passed
 - The running model is configured with `materialized='incremental'`