Rollup merge of #81465 - joshtriplett:duration-formatting-documentation, r=m-ou-se
Rollup merge of #81465 - joshtriplett:duration-formatting-documentation, r=m-ou-se

Add documentation about formatting `Duration` values

Explain why Duration has a Debug impl but not a Display impl, and
mention the use of Unicode.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust