DOC: Update mode parameter description to account for shape #22643 (#…

…22655)

The `shape` parameter must be specified when opened in appending mode. Docstring
and exception message wording are updated to reflect this.

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

numpy/core/memmap.py

@@ -59,6 +59,7 @@ class memmap(ndarray):
         | 'r+' | Open existing file for reading and writing.                 |
         +------+-------------------------------------------------------------+
         | 'w+' | Create or overwrite existing file for reading and writing.  |
+        |      | If ``mode == 'w+'`` then `shape` must also be specified.    |
         +------+-------------------------------------------------------------+
         | 'c'  | Copy-on-write: assignments affect data in memory, but       |
         |      | changes are not saved to disk.  The file on disk is         |
@@ -220,7 +221,7 @@ def __new__(subtype, filename, dtype=uint8, mode='r+', offset=0,
                 ) from None
 
         if mode == 'w+' and shape is None:
-            raise ValueError("shape must be given")
+            raise ValueError("shape must be given if mode == 'w+'")
 
         if hasattr(filename, 'read'):
             f_ctx = nullcontext(filename)