Slightly reduce I/O overhead in FSFS f7 when not run in block-read mode.
This gives it the same file access granularity as in older formats.

* subversion/libsvn_fs_fs/fs_fs.c
  (write_config): Update user doc in configuration template.
  (read_global_config): Cap block size to 4k.  Smaller values will still
                        be accepted.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606658 13f79535-47bb-0310-9956-ffa450edef68