FSFS: Improve the documentation of the filesystem's structure.
FSFS: Improve the documentation of the filesystem's structure.

* subversion/libsvn_fs_fs/structure
  (Layout): Mark directories, and document the existance of the mergeinfo
    database.  Fix grammar, and document the locking used for, the
    transaction-current file.
  (Filesystem formats): Document when the mergeinfo database and
    transaction-current files are present.
  (Transaction layout): Document the existence of the mergeinfo props, and
    document when the various prop files exist.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion