Update DAGCircuit.draw() docstring with current requirements (#11337)

* Update DAGCircuit.draw() docstring with current requirements

Since #8162 the dag drawer hasn't required pydot to run. It now uses
rustworkx's graphviz_draw() function which directly calls graphviz.
However, in #8162 the docstring for the DAGCircuit.draw() method was not
updated to reflect this and the method documentation still said that
pydot was required. This commit fixes this oversight and updates the
docstring to correctly state that only graphviz is required (as
rustworkx is a hard dependency for Qiskit it's not anything that needs
to be documented). It also includes more details on how to install
graphviz as this is often a potential source of confusion for users.

* Update qiskit/dagcircuit/dagcircuit.py

Co-authored-by: Jake Lishman <jake@binhbar.com>

---------

Co-authored-by: Jake Lishman <jake@binhbar.com>
(cherry picked from commit c3a7d40)

qiskit/dagcircuit/dagcircuit.py

@@ -2099,8 +2099,12 @@ def draw(self, scale=0.7, filename=None, style="color"):
         """
         Draws the dag circuit.
 
-        This function needs `pydot <https://github.com/erocarrera/pydot>`_, which in turn needs
-        `Graphviz <https://www.graphviz.org/>`_ to be installed.
+        This function needs `Graphviz <https://www.graphviz.org/>`_ to be
+        installed. Graphviz is not a python package and can't be pip installed
+        (the ``graphviz`` package on PyPI is a Python interface library for
+        Graphviz and does not actually install Graphviz). You can refer to
+        `the Graphviz documentation <https://www.graphviz.org/download/>`__ on
+        how to install it.
 
         Args:
             scale (float): scaling factor