overlayfs.rst: use consistent feature names

Use the feature names "metacopy" and "index" consistently throughout
the document.

Covert the numbered list of features "redirect_dir", "index", "xino"
to section headings, so that those features could be referenced in the
document by their name.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>