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

What are you changing in this pull request and why?

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

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'

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."
• The content in this PR requires a dbt release note, so I added one to the release notes page.

🚀 Deployment available! Here are the direct links to the updated files:

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