manpage: vo_opengl: describe how to get lanczos2/lanczos3

These aliases were removed in commit 1ec77214. Add a notice to the
manpage how to get these back. Apparently, "lanczos2" and "lanczos3"
were the only interesting aliases possibly used by someone, so the
description is limited to these two.

DOCS/man/vo.rst

@@ -303,6 +303,9 @@ Available video output drivers are:
             which is good for some content types. The number of taps can be
             controlled with ``scale-radius``, but is best left unchanged.
 
+            If the radius is not changed, this filter corresponds to the old
+            ``lanczos3`` alias, while ``lanczos2`` corresponds to a radius of 2.
+
         ``ewa_lanczos``
             Elliptic weighted average Lanczos scaling. Also known as Jinc.
             Relatively slow, but very good quality. The number of taps can