DOC: Update mode parameter description to account for shape #22643 (#22655)
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 <e9fb290bfd0a4406872fd6103425d9efafcfa051@berkeley.edu>
numpy/numpy,numpy/numpy,numpy/numpy,numpy/numpy