DOC: parser-iPythonCell.r / input.py: Note about the percent format

[westurner]

Possibly also worth noting somewhere that the linked docs have the percent format grammar as:

# %% Optional title [cell type] key="value"

https://jupytext.readthedocs.io/en/latest/formats-scripts.html#the-percent-format

[masatake]

I don't understand why adding such a comment to a test input is worth it. The worth is unclear to me. Users have no chance to see the comment.

[westurner]

Fair point.

Hadn't realized there's an .ipynb ctags parser. Now I'll remember.