Rollup merge of #29417 - pmarcelll:assert-doc, r=steveklabnik
Rollup merge of #29417 - pmarcelll:assert-doc, r=steveklabnik

…m message

I recently discovered that this is not mentioned in the docs, only in
the examples, and it's not evident for people coming from C++

r? @steveklabnik

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