Followup to r1573096, add some comments I should have added.

* subversion/libsvn_subr/sysinfo.c
  (write_to_cfmutabledata): Document.
  (system_version_plist): Explain why we're copying data around.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573098 13f79535-47bb-0310-9956-ffa450edef68