offapi: document queryLogicalAdvancements() for XTextLayout.

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>

offapi/com/sun/star/rendering/XTextLayout.idl

@@ -113,10 +113,14 @@ interface XTextLayout : ::com::sun::star::uno::XInterface
         This method returns a sequence of advancements, one for each
         character in the input string (<em>not</em> for every
         glyph. There might be multiple glyphs per input character, or
-        multiple input characters per glyph). Adding up all
-        advancements yields the total advancement of this layout. To
-        manipulate the layout of a string on the level of characters,
-        this method can be used to query for the layout's default
+        multiple input characters per glyph).
+
+        An advancement value is the distance of the glyph to the beginning
+        edge, which is left for LTR text and is right for RTL text. The
+        maximum of the advancements can be deemed as the width of the whole
+        text layout.
+
+        This method can be used to query for the layout's default
         advancements, which can subsequently be changed and applied to
         the layout via
         XTextLayout::applyLogicalAdvancements().<p>