Rollup merge of #59933 - sourcefrog:doc-fmt, r=shepmaster
Rollup merge of #59933 - sourcefrog:doc-fmt, r=shepmaster

Make clear that format padding doesn't work for Debug

As mentioned in
https://github.com/rust-lang/rust/issues/46006#issuecomment-345260633

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