docs: clarify scope of databricks_grant, reword reference to principals

[tlecomte]

Changes

Reword the following documentation pages:

• databricks_grant: it looks like the introduction text was copied from databricks_grants (plural), so reword to make it clearer that databricks_grant applies to a single securable+principal combination
• databricks_grant and databricks_grants: a minor reword on users and groups are referred to as principals

Tests

Look at the markdown renders.

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Inputs:

• PR number: 5182
• Commit SHA: 2012bde856b1044586b60901fc391eb97f663415

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