Rollup merge of #25087 - nham:improve_vec_docs, r=Gankro
Rollup merge of #25087 - nham:improve_vec_docs, r=Gankro

 This commit does two things: it adds an example for indexing vectors, and it changes the \"Examples\" section to use full sentences.

This change was spurred by someone in the #rust IRC channel asking if there was a `.set()` method for changing the `i`-th value of a vector (they had missed that `Vec` implements `IndexMut`, which is easy to do if you're not aware of that trait).

omasanori/rust,seanrivera/rust,l0kod/rust,vhbit/rust,mdinger/rust,graydon/rust,rohitjoshi/rust,graydon/rust,ejjeong/rust,victorvde/rust,sae-bom/rust,mvdnes/rust,carols10cents/rust,omasanori/rust,jashank/rust,aepsil0n/rust,graydon/rust,mihneadb/rust,GBGamer/rust,krzysz00/rust,sae-bom/rust,jroesch/rust,ejjeong/rust,KokaKiwi/rust,mihneadb/rust,zubron/rust,l0kod/rust,aepsil0n/rust,nwin/rust,ejjeong/rust,kwantam/rust,nwin/rust,aneeshusa/rust,dwillmer/rust,gifnksm/rust,andars/rust,reem/rust,jroesch/rust,jroesch/rust,ejjeong/rust,pshc/rust,vhbit/rust,jashank/rust,GBGamer/rust,aidancully/rust,sae-bom/rust,omasanori/rust,graydon/rust,omasanori/rust,pshc/rust,aepsil0n/rust,kwantam/rust,cllns/rust,andars/rust,jashank/rust,aidancully/rust,rohitjoshi/rust,pshc/rust,pelmers/rust,mihneadb/rust,robertg/rust,vhbit/rust,GBGamer/rust,robertg/rust,TheNeikos/rust,carols10cents/rust,aepsil0n/rust,jroesch/rust,seanrivera/rust,philyoon/rust,aidancully/rust,krzysz00/rust,jroesch/rust,zachwick/rust,gifnksm/rust,KokaKiwi/rust,l0kod/rust,zubron/rust,nwin/rust,seanrivera/rust,jroesch/rust,andars/rust,gifnksm/rust,reem/rust,philyoon/rust,zachwick/rust,jashank/rust,dwillmer/rust,pelmers/rust,mdinger/rust,jashank/rust,mdinger/rust,aidancully/rust,zachwick/rust,aneeshusa/rust,GBGamer/rust,aneeshusa/rust,reem/rust,cllns/rust,sae-bom/rust,ejjeong/rust,pelmers/rust,jashank/rust,carols10cents/rust,dwillmer/rust,zubron/rust,nwin/rust,kwantam/rust,pshc/rust,seanrivera/rust,mdinger/rust,zubron/rust,pshc/rust,zubron/rust,aidancully/rust,vhbit/rust,aneeshusa/rust,GBGamer/rust,vhbit/rust,zachwick/rust,andars/rust,pelmers/rust,pshc/rust,sae-bom/rust,gifnksm/rust,pelmers/rust,andars/rust,reem/rust,pelmers/rust,kwantam/rust,KokaKiwi/rust,rohitjoshi/rust,XMPPwocky/rust,robertg/rust,zubron/rust,mihneadb/rust,krzysz00/rust,victorvde/rust,XMPPwocky/rust,krzysz00/rust,carols10cents/rust,nwin/rust,dwillmer/rust,KokaKiwi/rust,XMPPwocky/rust,gifnksm/rust,victorvde/rust,dwillmer/rust,vhbit/rust,krzysz00/rust,rohitjoshi/rust,mvdnes/rust,robertg/rust,kwantam/rust,seanrivera/rust,omasanori/rust,aepsil0n/rust,vhbit/rust,mvdnes/rust,rohitjoshi/rust,mvdnes/rust,XMPPwocky/rust,KokaKiwi/rust,l0kod/rust,aneeshusa/rust,cllns/rust,GBGamer/rust,nwin/rust,cllns/rust,philyoon/rust,graydon/rust,TheNeikos/rust,TheNeikos/rust,mdinger/rust,jashank/rust,TheNeikos/rust,aidancully/rust,krzysz00/rust,mvdnes/rust,victorvde/rust,aepsil0n/rust,carols10cents/rust,TheNeikos/rust,mvdnes/rust,philyoon/rust,GBGamer/rust,l0kod/rust,cllns/rust,pshc/rust,reem/rust,GBGamer/rust,dwillmer/rust,mdinger/rust,zachwick/rust,seanrivera/rust,victorvde/rust,l0kod/rust,kwantam/rust,philyoon/rust,vhbit/rust,andars/rust,reem/rust,sae-bom/rust,omasanori/rust,robertg/rust,graydon/rust,zubron/rust,rohitjoshi/rust,cllns/rust,zachwick/rust,gifnksm/rust,TheNeikos/rust,XMPPwocky/rust,jroesch/rust,zubron/rust,dwillmer/rust,jroesch/rust,carols10cents/rust,nwin/rust,ejjeong/rust,mihneadb/rust,aneeshusa/rust,robertg/rust,XMPPwocky/rust,victorvde/rust,jashank/rust,philyoon/rust,l0kod/rust,l0kod/rust,mihneadb/rust,nwin/rust,dwillmer/rust,KokaKiwi/rust,pshc/rust