In the FSFS index code and structure docs, make it explicit
that empty P2L index pages are a thing.

* subversion/libsvn_fs_fs/index.c
  (get_p2l_page): Make empty pages an explicitly handled
                  special case.

* subversion/libsvn_fs_fs/structure-indexes
  (P2L / Index on-disk format): Explicitly state that large
                                items cause zero entry pages.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1633617 13f79535-47bb-0310-9956-ffa450edef68