Bug 4877: Add missing text about external_acl_type %DATA changes (#276)

Bug 4877: Add missing text about external_acl_type %DATA changes (#276)

Conversion of external_acl_type to using logformat macros was
not quite seamless. The %DATA macro now expands to a dash '-' to
fix helpers using it explicitly from receiving incorrect number
of fields (and misaligned input) on their input lines.

Unfortunately that also results in the implicit use of that
macro expanding to non-whitespace ('-'). That small fact was not
documented in the initial v4 release notes and config texts.