Slightly misleading chrono extension documentation

[eli-b]

The Chrono Format Specifications section claims "the available presentation types ... for chrono durations and time points are" and then lists only 'H', 'M', and 'S'. This may lead the reader to believe the rest of the ~39 chrono_type characters only apply to std::tm, which I believe is incorrect.

Also, following #1866, the list of chrono presentation types should be expanded with a description for each type. If the types are meant to be the same as strftime's, this is not mentioned.

[vitaut]

The docs have been updated: https://fmt.dev/dev/syntax.html#chrono-specs.

[eli-b]

Thanks, @vitaut, this is a great improvement to the docs! And so quick, too...