Changed the tooltip description for uniqueString() to include length

[StefanIvemo]

Fixes #7333

Changed the tooltip description for uniqueString():

From: Creates a deterministic hash string based on the values provided as parameters.

To: Creates a deterministic hash string based on the values provided as parameters. The returned value is 13 characters long.

Contributing a feature

• I have opened a new issue for the proposal, or commented on an existing one, and ensured that the Bicep maintainers are good with the design of the feature being implemented
• I have included "Fixes #{issue_number}" in the PR description, so GitHub can link to the issue and close it when the PR is merged
• I have appropriate test coverage of my new feature

Microsoft Reviewers: Open in CodeFlow

[anthony-c-martin]

Thanks for fixing this!