Add documentation for use of proprietary licence (#2231)

Resolves: #2198

docs/docs/pyproject.md

@@ -40,6 +40,10 @@ The recommended notation for the most common licenses is (alphabetical):
 Optional, but it is highly recommended to supply this.
 More identifiers are listed at the [SPDX Open Source License Registry](https://www.spdx.org/licenses/).
 
+!!!note
+
+    If your project is proprietary and does not use a specific licence, you can set this value as `Proprietary`.
+
 ## authors
 
 The authors of the package. **Required**