DOC: Clarify contour levels parameter as boundaries to fix ambiguity

The previous documentation stated that `MaxNLocator` chooses "no more than n+1" levels, which was mathematically incorrect for the default case (n=7 produces 9 levels).

Additionally, the distinction between `contour` (lines) and `contourf` (filled regions) created confusion, as `contourf` produces one fewer region than the number of lines (unless `extend` is used).

This update redefines `levels` strictly as "contour boundaries." This terminology:
1. Is accurate for both functions (lines are at boundaries, regions are between them).
2. Remains correct regardless of the `extend` parameter.
3. Explicitly documents the default behavior for n=7.

Author: sanrishi

lib/matplotlib/contour.py

@@ -1487,15 +1487,17 @@ def _initialize_x_y(self, z):
     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+2* "nice" contour levels
-    between the minimum and maximum numeric values of *Z*.
+    automatically choose no more than *n+2* "nice" contour level boundaries
+    between the minimum and maximum numeric values of *Z*. These boundaries
+    define where lines are drawn (for `contour`) or where filled regions
+    are separated (for `contourf`).
 
     If array-like, draw contour lines at the specified levels.
     The values must be in increasing order.
 
-    If not given, a reasonable default is automatically chosen. For linear
-    scales, `~matplotlib.ticker.MaxNLocator` is used with *nbins*=8,
-    producing up to 9 "nice" contour levels. For logarithmic scales,
+    If not specified, a reasonable default is automatically chosen. For
+    linear scales, this corresponds to *n=7* (using `~.ticker.MaxNLocator(8)`),
+    producing up to 9 "nice" contour boundaries. For logarithmic scales,
     `~matplotlib.ticker.LogLocator` is used instead.
 
 Returns