send_key inline documentation contains less atoms than key_codes_json can handle

[sphaso]

Reading the documentation I was puzzled by the absence of some atoms (e.g. arrow down) for the Page.send_key function, only to find they are actually handled when digging into the code.
Is there a reason why some keys are not documented? If not, I would love to send a pull request updating the send_key inline documentation to match the InternalHelpers.key_codes_json pattern matchings.