Rollup merge of #48738 - Songbird0:parseinterror_potential_cause, r=joshtriplett
Rollup merge of #48738 - Songbird0:parseinterror_potential_cause, r=joshtriplett

Add a potential cause raising `ParseIntError`.

Initially, I wanted to add it directly to the documentation of `str. parse()` method, I finally found that it was more relevant (I hope so?) to directly document the structure in question. I've added a scenario, in which we could all get caught at least once, to make it easier to diagnose the problem when parsing integers.

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