Improve pytest_plugins docs
Improve pytest_plugins docs

As discussed in #2246
hackebrot/pytest,txomon/pytest,Akasurde/pytest,The-Compiler/pytest,nicoddemus/pytest,tgoodlet/pytest,MichaelAquilina/pytest,rmfitzpatrick/pytest,vmalloc/dessert,The-Compiler/pytest,davidszotten/pytest,tomviner/pytest,markshao/pytest,pytest-dev/pytest,skylarjhdownes/pytest,pfctdayelise/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,eli-b/pytest,flub/pytest,ddboline/pytest,alfredodeza/pytest,jaraco/pytest,etataurov/pytest,nicoddemus/pytest,tareqalayan/pytest