Auto merge of #27012 - pornel:master, r=Gankro
Auto merge of #27012 - pornel:master, r=Gankro

Fixes #26689

This PR tries to clarify uses of "character" where it means "code point" or "UTF-8 sequence", which are almost, but not quite the same. Edge cases added to some examples to demonstrate this.

However, I've kept use of the term "code point" instead of "Unicode scalar value", because in UTF-8 they're the same, and "code point" is more widely known.

jashank/rust,jroesch/rust,dwillmer/rust,jashank/rust,GBGamer/rust,aidancully/rust,zubron/rust,pshc/rust,pshc/rust,jroesch/rust,jroesch/rust,l0kod/rust,vhbit/rust,vhbit/rust,l0kod/rust,zubron/rust,pshc/rust,pshc/rust,GBGamer/rust,jashank/rust,zubron/rust,aidancully/rust,l0kod/rust,dwillmer/rust,vhbit/rust,vhbit/rust,l0kod/rust,l0kod/rust,l0kod/rust,vhbit/rust,dwillmer/rust,jashank/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,zubron/rust,jashank/rust,aidancully/rust,jroesch/rust,vhbit/rust,aidancully/rust,jashank/rust,dwillmer/rust,graydon/rust,GBGamer/rust,jroesch/rust,pshc/rust,jroesch/rust,vhbit/rust,jroesch/rust,l0kod/rust,GBGamer/rust,GBGamer/rust,jashank/rust,graydon/rust,vhbit/rust,GBGamer/rust,dwillmer/rust,jroesch/rust,GBGamer/rust,zubron/rust,dwillmer/rust,dwillmer/rust,pshc/rust,dwillmer/rust,zubron/rust,jashank/rust,pshc/rust,l0kod/rust,graydon/rust,zubron/rust,zubron/rust,graydon/rust,GBGamer/rust,pshc/rust