Based on implementation, explain the definition of logical
advancements for RTL and LTR case and remove incorrect part.
Logical advancements are distances to the edge instead of
widths of characters, so summing all do not yield the
total.

Change-Id: I437f490e5025c2124238e8cad506a966d7bc792c
Reviewed-on: https://gerrit.libreoffice.org/61461
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>