Improve docstrings for Approx classes.
Improve docstrings for Approx classes.

alfredodeza/pytest,Akasurde/pytest,markshao/pytest,nicoddemus/pytest,ddboline/pytest,hackebrot/pytest,tomviner/pytest,nicoddemus/pytest,The-Compiler/pytest,davidszotten/pytest,pfctdayelise/pytest,txomon/pytest,pytest-dev/pytest,The-Compiler/pytest,RonnyPfannschmidt/pytest,tomviner/pytest