[krbdev.mit.edu #5279] Doc improvement

[Jeff Blaine via RT]

http://web.mit.edu/Kerberos/krb5-1.5/krb5-1.5.1/doc/krb5-install/Add-Administrators-to-the-Acl-File.html#Add%20Administrators%20to%20the%20Acl%20File

IMO, should start with an opening paragraph that indicates what
the ACL file is for, what references it, etc...

"In order to control access to your KDC and its databases, you
must configure an Access Control List (ACL) file with at least
1 administrator and his/her permissions defined."