Bug 7413: (follow-up) tweak description of the AllowRenewalLimitOverride syspref

Bug 7413: (follow-up) tweak description of the AllowRenewalLimitOverride syspref

As it happens, the AllowRenewalLimitOverride serves to both allow
operators to override renewal blocks when the renewal exceed the
maximum renewal count or when the renewal is premature.  This
patch adjust the wording to reflect the expansion in meaning; if
it turns out that folks want to distinguish between overriding the
maximum renewal count and overriding premature renewals, a new
system preference can be added later.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>