Re #16480 add documentation for UserFunctionMD

docs/source/fitting/fitfunctions/UserFunctionMD.rst

+.. _func-UserFunctionMD:
+
+==============
+UserFunctionMD
+==============
+
+.. index:: UserFunctionMD
+
+Description
+-----------
+
+A UserFunctionMD is the multi-dimensional equivalent of :ref:`UserFunction <func-UserFunction>`. It is defined by a string formula.
+The formula is assigned by setting string attribute Formula:
+
+`` "name=UserFunctionMD,Formula=a + y + (b + 0.1*y) * x, a=0, b=2"``
+
+The fitting parameters become defined only after the Formula attribute is set that is why Formula must
+go first in UserFunctionMD definition.
+
+.. attributes::
+
+.. properties::
+
+.. categories::
+
+.. sourcelink::
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