Add doctests for union, diff, sym_diff, and intersection.
Add doctests for union, diff, sym_diff, and intersection.

Add a rustdoc test for union to exhibit how it is used.

There is already a test for union in the test namespace, but this commit
adds a doctest that will appear in the rustdocs.

Add a doctest for the difference function.

Add a doctest for the symmetric_difference function.

Add a doctest for the intersection function.

Update the union et al. doctests based on @Gankro's comments.

Make the union et al. doctests a bit more readable.

aidancully/rust,GBGamer/rust,omasanori/rust,sae-bom/rust,aidancully/rust,kwantam/rust,zubron/rust,aidancully/rust,dwillmer/rust,aneeshusa/rust,zaeleus/rust,dinfuehr/rust,mahkoh/rust,TheNeikos/rust,rprichard/rust,zaeleus/rust,avdi/rust,kimroen/rust,GBGamer/rust,zachwick/rust,mvdnes/rust,emk/rust,kwantam/rust,richo/rust,KokaKiwi/rust,miniupnp/rust,nwin/rust,philyoon/rust,miniupnp/rust,graydon/rust,l0kod/rust,cllns/rust,mvdnes/rust,andars/rust,seanrivera/rust,untitaker/rust,cllns/rust,GBGamer/rust,mihneadb/rust,zaeleus/rust,GBGamer/rust,aidancully/rust,sae-bom/rust,pshc/rust,reem/rust,pshc/rust,emk/rust,reem/rust,ruud-v-a/rust,avdi/rust,AerialX/rust-rt-minimal,carols10cents/rust,avdi/rust,nwin/rust,rohitjoshi/rust,AerialX/rust,ejjeong/rust,untitaker/rust,vhbit/rust,rohitjoshi/rust,gifnksm/rust,philyoon/rust,robertg/rust,TheNeikos/rust,TheNeikos/rust,XMPPwocky/rust,aneeshusa/rust,rohitjoshi/rust,jashank/rust,zaeleus/rust,emk/rust,emk/rust,GBGamer/rust,robertg/rust,bombless/rust,miniupnp/rust,jroesch/rust,hauleth/rust,dwillmer/rust,zachwick/rust,krzysz00/rust,vhbit/rust,GBGamer/rust,miniupnp/rust,AerialX/rust,mahkoh/rust,nwin/rust,l0kod/rust,AerialX/rust,ebfull/rust,krzysz00/rust,avdi/rust,rprichard/rust,gifnksm/rust,zubron/rust,dinfuehr/rust,zubron/rust,dinfuehr/rust,rprichard/rust,seanrivera/rust,mdinger/rust,cllns/rust,dinfuehr/rust,victorvde/rust,omasanori/rust,l0kod/rust,dwillmer/rust,zubron/rust,seanrivera/rust,nwin/rust,kimroen/rust,zaeleus/rust,bombless/rust,jroesch/rust,seanrivera/rust,mihneadb/rust,zubron/rust,victorvde/rust,dwillmer/rust,jashank/rust,hauleth/rust,emk/rust,jroesch/rust,seanrivera/rust,rohitjoshi/rust,pelmers/rust,andars/rust,richo/rust,l0kod/rust,defuz/rust,rohitjoshi/rust,vhbit/rust,avdi/rust,jroesch/rust,mihneadb/rust,jashank/rust,carols10cents/rust,mdinger/rust,ejjeong/rust,GBGamer/rust,hauleth/rust,AerialX/rust-rt-minimal,jroesch/rust,ruud-v-a/rust,zubron/rust,l0kod/rust,hauleth/rust,AerialX/rust,ruud-v-a/rust,victorvde/rust,bombless/rust,bombless/rust,mdinger/rust,mvdnes/rust,pelmers/rust,sae-bom/rust,AerialX/rust-rt-minimal,ktossell/rust,AerialX/rust,gifnksm/rust,bombless/rust-docs-chinese,robertg/rust,victorvde/rust,defuz/rust,jroesch/rust,gifnksm/rust,AerialX/rust-rt-minimal,jashank/rust,jashank/rust,l0kod/rust,ruud-v-a/rust,jroesch/rust,nwin/rust,andars/rust,sae-bom/rust,robertg/rust,kimroen/rust,ejjeong/rust,zubron/rust,zachwick/rust,XMPPwocky/rust,graydon/rust,zachwick/rust,mahkoh/rust,robertg/rust,omasanori/rust,rprichard/rust,TheNeikos/rust,ktossell/rust,kwantam/rust,philyoon/rust,kimroen/rust,krzysz00/rust,zubron/rust,miniupnp/rust,jroesch/rust,miniupnp/rust,aepsil0n/rust,aepsil0n/rust,zachwick/rust,richo/rust,defuz/rust,defuz/rust,jashank/rust,l0kod/rust,philyoon/rust,defuz/rust,untitaker/rust,AerialX/rust-rt-minimal,victorvde/rust,gifnksm/rust,reem/rust,pshc/rust,pelmers/rust,cllns/rust,AerialX/rust,mahkoh/rust,reem/rust,ebfull/rust,dinfuehr/rust,mvdnes/rust,aneeshusa/rust,pelmers/rust,untitaker/rust,defuz/rust,andars/rust,ebfull/rust,GBGamer/rust,pshc/rust,rohitjoshi/rust,dwillmer/rust,ejjeong/rust,victorvde/rust,emk/rust,reem/rust,aneeshusa/rust,hauleth/rust,aidancully/rust,aepsil0n/rust,carols10cents/rust,jashank/rust,andars/rust,pshc/rust,kwantam/rust,mdinger/rust,carols10cents/rust,pshc/rust,dwillmer/rust,mdinger/rust,ebfull/rust,graydon/rust,dinfuehr/rust,XMPPwocky/rust,ktossell/rust,KokaKiwi/rust,rprichard/rust,kimroen/rust,ktossell/rust,omasanori/rust,richo/rust,aneeshusa/rust,ktossell/rust,ebfull/rust,cllns/rust,pelmers/rust,nwin/rust,vhbit/rust,jashank/rust,omasanori/rust,kwantam/rust,richo/rust,mahkoh/rust,emk/rust,ruud-v-a/rust,graydon/rust,gifnksm/rust,richo/rust,XMPPwocky/rust,pshc/rust,vhbit/rust,aepsil0n/rust,aneeshusa/rust,carols10cents/rust,TheNeikos/rust,ejjeong/rust,nwin/rust,KokaKiwi/rust,cllns/rust,ejjeong/rust,krzysz00/rust,vhbit/rust,KokaKiwi/rust,untitaker/rust,ktossell/rust,zachwick/rust,ebfull/rust,kwantam/rust,miniupnp/rust,seanrivera/rust,hauleth/rust,krzysz00/rust,zaeleus/rust,kimroen/rust,miniupnp/rust,untitaker/rust,dwillmer/rust,mahkoh/rust,bombless/rust,KokaKiwi/rust,avdi/rust,kimroen/rust,bombless/rust,rprichard/rust,l0kod/rust,mihneadb/rust,reem/rust,ruud-v-a/rust,aepsil0n/rust,KokaKiwi/rust,carols10cents/rust,mihneadb/rust,dwillmer/rust,AerialX/rust-rt-minimal,mdinger/rust,krzysz00/rust,mihneadb/rust,mvdnes/rust,vhbit/rust,pshc/rust,mvdnes/rust,vhbit/rust,philyoon/rust,omasanori/rust,TheNeikos/rust,aepsil0n/rust,ktossell/rust,graydon/rust,philyoon/rust,sae-bom/rust,andars/rust,graydon/rust,aidancully/rust,XMPPwocky/rust,nwin/rust,pelmers/rust,sae-bom/rust,robertg/rust,XMPPwocky/rust