Edited note on :class:.NumberPlane length and added another example (#2726)

* docs: edit note on lengths and add example for it

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* Update manim/mobject/graphing/coordinate_systems.py

Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

* Update manim/mobject/graphing/coordinate_systems.py

Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

Author: 3 people

manim/mobject/graphing/coordinate_systems.py

@@ -2228,8 +2228,8 @@ class NumberPlane(Axes):
 
 
     .. note::
-        If :attr:`x_length` or :attr:`y_length` are not defined, the plane automatically adjusts its lengths based
-        on the :attr:`x_range` and :attr:`y_range` values to set the ``unit_size`` to 1.
+        If :attr:`x_length` or :attr:`y_length` are not defined, they are automatically calculated such that
+        one unit on each axis is one Manim unit long.
 
     Examples
     --------
@@ -2239,15 +2239,34 @@ class NumberPlane(Axes):
         class NumberPlaneExample(Scene):
             def construct(self):
                 number_plane = NumberPlane(
-                    x_range=[-10, 10, 1],
-                    y_range=[-10, 10, 1],
                     background_line_style={
                         "stroke_color": TEAL,
                         "stroke_width": 4,
                         "stroke_opacity": 0.6
                     }
                 )
                 self.add(number_plane)
+
+    .. manim:: NumberPlaneScaled
+        :save_last_frame:
+
+        class NumberPlaneScaled(Scene):
+            def construct(self):
+                number_plane = NumberPlane(
+                    x_range=(-4, 11, 1),
+                    y_range=(-3, 3, 1),
+                    x_length=5,
+                    y_length=2,
+                ).move_to(LEFT*3)
+
+                number_plane_scaled_y = NumberPlane(
+                    x_range=(-4, 11, 1),
+                    x_length=5,
+                    y_length=4,
+                ).move_to(RIGHT*3)
+
+                self.add(number_plane)
+                self.add(number_plane_scaled_y)
     """
 
     def __init__(