Add clarity to docs on permalinks placeholders and builtins (#8995)
Add clarity to docs on permalinks placeholders and builtins (#8995)

Merge pull request 8995
chrisfinazzo/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,jekyll/jekyll,jekyll/jekyll,jekyll/jekyll,chrisfinazzo/jekyll