Doc: Clarify default levels behavior in contour/contourf

Explicitly state that the default levels corresponds to n=7 and clarifies that int input targets n+1 levels but may produce n+2.

Author: sanrishi

lib/matplotlib/contour.py

@@ -1485,13 +1485,17 @@ def _initialize_x_y(self, z):
 
 levels : int or array-like, optional
     Determines the number and positions of the contour lines / regions.
-
-    If an int *n*, use `~matplotlib.ticker.MaxNLocator`, which tries
-    to automatically choose no more than *n+1* "nice" contour levels
-    between minimum and maximum numeric values of *Z*.
-
+    
+    If an int *n*, use `~matplotlib.ticker.MaxNLocator` to automatically
+    choose approximately *n+1* "nice" contour levels between the minimum
+    and maximum values of *Z*. Note that the actual number may be up to
+    *n+2* depending on the data range.
+    
     If array-like, draw contour lines at the specified levels.
     The values must be in increasing order.
+    
+    If not specified, behaves as if ``levels=7`` was passed, typically
+    producing approximately 8 contour levels.
 
 Returns
 -------