Improve docs for yield-fixture and with statement a bit
Improve docs for yield-fixture and with statement a bit

Fix #2262

rmfitzpatrick/pytest,davidszotten/pytest,tomviner/pytest,flub/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest,alfredodeza/pytest,Akasurde/pytest,nicoddemus/pytest,txomon/pytest,MichaelAquilina/pytest,tomviner/pytest,tareqalayan/pytest,nicoddemus/pytest,etataurov/pytest,jaraco/pytest,markshao/pytest,pytest-dev/pytest,ddboline/pytest,pfctdayelise/pytest,skylarjhdownes/pytest,tgoodlet/pytest,hackebrot/pytest,The-Compiler/pytest,vmalloc/dessert