doc: adjust chrono_format_spec documentation to match library behavior

[jengelh]

References: #3237

[vitaut]

Thanks for the PR. We shouldn't remove the part of the grammar because it is valid for durations. I suggest clarifying that time points only support chrono_specs at the moment in a sentence below the grammar.

[jengelh]

Since durations don't do conversion ({:%d}), I've changed the syntax for durations to not include conversion_spec anymore.

[vitaut]

Durations do support conversion specifiers except calendaric so please revert that change. We already explain the calendaric part in the docs.

[vitaut]

@jengelh do you still plan to update this PR per my earlier comment?

[jengelh]

I don't know how to dismiss this "Changes requested" box - GH does not notice the new change and does not show a "Done" button or something that I could have sworn existed elsewhere. whatever

[vitaut]

This is incorrect because durations support conversion specifiers. Let's correct it by removing this line which will also fix duplicate definition of chrono_specs.

still needs changes

[vitaut]

This is no longer needed because time points are getting missing format specifier support in #3260 but thanks anyway.