Remove Private Methods/Properties From Online Doc

Documenting public and protected methods is useful; I don't believe it is useful to document private methods. If people really need them, they can clone the project and run PhpDocumentor against it.

As long as I'm at it, we are several releases behind in PhpDocumentor. Use the latest (3.8.1 replacing 3.5.0). I have confirmed that this works locally on my machine. There's no real way to verify the change until the next PhpSpreadsheet release. Hope it works.

Author: oleibman

.github/workflows/github-pages.yml

@@ -23,8 +23,8 @@ jobs:
 
       - name: Build API documentation
         run: |
-          curl -LO https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.5.0/phpDocumentor.phar
-          php phpDocumentor.phar --directory src/ --target docs/api
+          curl -LO https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.8.1/phpDocumentor.phar
+          php phpDocumentor.phar --directory src/ --target docs/api --visibility=public,protected
 
       - name: Deploy to GithHub Pages
         uses: peaceiris/actions-gh-pages@v3