Rewrite explanation for \anchorborder

doc/generic/pgf/text-en/pgfmanual-en-base-nodes.tex

@@ -1100,11 +1100,21 @@ \subsubsection{Command for Declaring New Shapes}
         |\pgfpointshapeborder| command, the \meta{code} will be executed to
         discern this point. When the execution of  the \meta{code} starts, the
         dimensions |\pgf@x| and |\pgf@y| will have been set to a location $p$
-        in the shape's coordinate system. It is now the job of the \meta{code}
-        to set up |\pgf@x| and |\pgf@y| such that they specify the point on the
-        shape's border that lies on a straight line from the shape's center to
-        the point $p$. Usually, this is a somewhat complicated computation,
-        involving many case distinctions and some basic math.
+        in the shape's coordinate system, and relative to the anchor |center|.
+        Note that |\pgfpointshapeborder| will produce an error if the shape does
+        not contain the |center| anchor.
+
+        It is now the job of the \meta{code} to set up |\pgf@x| and |\pgf@y|
+        such that they specify the point on the shape's border that lies on a
+        straight line from the shape's center to the point $p$. Usually, this is
+        a somewhat complicated computation, involving many case distinctions and
+        some basic math. Note that the output coordinates must be returned in the
+        shape's coordinate system, \emph{no longer} relative to the |center|
+        anchor. While these different points of reference are only noticeable if
+        the |center| anchor is not at the origin of the shape's coordinate system,
+        it implies that ``doing nothing'' as a border anchor, i.e., returning the
+        point that was fed to |\pgfpointshapeborder| requires adding the |center|
+        anchor to the input coordinates.
 
         For our |simple rectangle| we must compute a point on the border of a
         rectangle whose one corner is the origin (ignoring the depth for