Add more detail about the zoom level of detail

Co-Authored-By: Yvonne Fröhlich <94163266+yvonnefroehlich@users.noreply.github.com>

Author: weiji14

pygmt/datasets/tile_map.py

@@ -29,8 +29,17 @@ def load_tile_map(region, zoom="auto", source=None, lonlat=True, wait=0, max_ret
         *ymin*, *ymax*]. These coordinates should be in longitude/latitude if
         ``lonlat=True`` or Spherical Mercator (EPSG:3857) if ``lonlat=False``.
 
-    zoom : int
-        Optional. Level of detail. [Default is 'auto'].
+    zoom : int or str
+        Optional. Level of detail. Higher levels (e.g. ``22``) means a zoom
+        level closer to the Earth's surface, with more tiles covering a smaller
+        geographical area and thus more detail. Lower levels (e.g. ``0``) means
+        a zoom level further from the Earth's surface, with less tiles covering
+        a larger geographical area and thus less detail. [Default is
+        ``"auto"`` to automatically determine the zoom level based on the
+        bounding box region extent].
+
+        Note: the maximum possible zoom level may be smaller than 22, and
+        depends on what is supported by the chosen web tile provider source.
 
     source : xyzservices.TileProvider or str
         Optional. The tile source: web tile provider or path to a local file.