Added links to the mosaic docs in figure and pyplot module docstrings (matplotlib#21546)

* Added links to the mosaic docs in figure and pyplot module docstrings

* I got rid of some whitespaces and dropped the fileending at the end of the link

* I changed the comment a bit in the docstrings

* I changed the comment describing the purpose of the link

Author: Sven Eschlbeck

lib/matplotlib/figure.py

@@ -1679,6 +1679,9 @@ def subplot_mosaic(self, mosaic, *, sharex=False, sharey=False,
            This API is provisional and may be revised in the future based on
            early user feedback.
 
+        See :doc:`/tutorials/provisional/mosaic`
+        for an example and full API documentation
+
         Parameters
         ----------
         mosaic : list of list of {hashable or nested} or str

lib/matplotlib/pyplot.py

@@ -1451,6 +1451,9 @@ def subplot_mosaic(mosaic, *, sharex=False, sharey=False,
        This API is provisional and may be revised in the future based on
        early user feedback.
 
+    See :doc:`/tutorials/provisional/mosaic`
+    for an example and full API documentation
+
     Parameters
     ----------
     mosaic : list of list of {hashable or nested} or str