Be more specific about the units used in FSFS config data.

Mention units explicitly in the FSFS config file template
and add commentary to our internal data structures.

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t): Internal sizes are in bytes.

* subversion/libsvn_fs_fs/fs_fs.c
  (read_config): Add commentary to the unit conversions.
                 Explain the pre-f7 dummy values.
  (write_config): Sizes in the config template are in kBytes.

Suggested by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616613 13f79535-47bb-0310-9956-ffa450edef68