docs: split rows share one y axis

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

help/C/org.udpscope.app/cursors.page

@@ -44,7 +44,8 @@
       any row moves it on all of them. That is what makes <code>dt</code> meaningful across
       rows.</p></item>
       <item><p>A ruler belongs to its row and nothing shares it, so two rows' rulers are
-      independent.</p></item>
+      independent. The plots of a split row are one row here: they share their y axis, so the
+      rulers are at the same values in each.</p></item>
       <item><p><gui>mix</gui> is the reading mode: one line per <em>bound</em> trace, with the
       trace's own values at each marker and their difference. A slot with a name but no
       channel behind it is not a trace and gets no line.</p></item>