DOC: spell out difference between source and target
DOC: spell out difference between source and target

The existing documentation would either not make it clear what vertices are considered sources, and which are targets, or it would be inconsistent and swap the roles of sources and targets (cf. https://github.com/scipy/scipy/issues/10733). This streamlines the documentation accordingly.

In the process, we also remove a few redundant spaces, and improve formatting slightly.
scipy/scipy,tylerjereddy/scipy,tylerjereddy/scipy,zerothi/scipy,zerothi/scipy,pizzathief/scipy,matthew-brett/scipy,vigna/scipy,zerothi/scipy,ilayn/scipy,Stefan-Endres/scipy,jor-/scipy,mdhaber/scipy,anntzer/scipy,Eric89GXL/scipy,jamestwebber/scipy,jamestwebber/scipy,grlee77/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,ilayn/scipy,endolith/scipy,person142/scipy,e-q/scipy,aarchiba/scipy,jor-/scipy,jamestwebber/scipy,anntzer/scipy,aarchiba/scipy,ilayn/scipy,arokem/scipy,grlee77/scipy,zerothi/scipy,grlee77/scipy,anntzer/scipy,andyfaff/scipy,matthew-brett/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,ilayn/scipy,Eric89GXL/scipy,WarrenWeckesser/scipy,mdhaber/scipy,Stefan-Endres/scipy,pizzathief/scipy,Eric89GXL/scipy,arokem/scipy,person142/scipy,perimosocordiae/scipy,nmayorov/scipy,pizzathief/scipy,aeklant/scipy,mdhaber/scipy,vigna/scipy,matthew-brett/scipy,nmayorov/scipy,tylerjereddy/scipy,pizzathief/scipy,scipy/scipy,endolith/scipy,rgommers/scipy,scipy/scipy,mdhaber/scipy,vigna/scipy,Stefan-Endres/scipy,scipy/scipy,pizzathief/scipy,rgommers/scipy,andyfaff/scipy,rgommers/scipy,arokem/scipy,WarrenWeckesser/scipy,arokem/scipy,e-q/scipy,aarchiba/scipy,person142/scipy,nmayorov/scipy,Stefan-Endres/scipy,jor-/scipy,Eric89GXL/scipy,perimosocordiae/scipy,endolith/scipy,Stefan-Endres/scipy,scipy/scipy,zerothi/scipy,perimosocordiae/scipy,andyfaff/scipy,Eric89GXL/scipy,scipy/scipy,aeklant/scipy,e-q/scipy,aeklant/scipy,ilayn/scipy,ilayn/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,endolith/scipy,rgommers/scipy,e-q/scipy,endolith/scipy,mdhaber/scipy,vigna/scipy,vigna/scipy,WarrenWeckesser/scipy,mdhaber/scipy,perimosocordiae/scipy,perimosocordiae/scipy,anntzer/scipy,anntzer/scipy,aeklant/scipy,jamestwebber/scipy,aarchiba/scipy,matthew-brett/scipy,andyfaff/scipy,rgommers/scipy,jor-/scipy,perimosocordiae/scipy,andyfaff/scipy,grlee77/scipy,anntzer/scipy,jor-/scipy,aeklant/scipy,tylerjereddy/scipy,zerothi/scipy,person142/scipy,grlee77/scipy,jamestwebber/scipy,matthew-brett/scipy,person142/scipy,endolith/scipy,arokem/scipy,nmayorov/scipy,e-q/scipy,andyfaff/scipy,aarchiba/scipy,nmayorov/scipy