DOC: Improve assert_warns docstring with example
DOC: Improve assert_warns docstring with example

jakirkham/numpy,mhvk/numpy,anntzer/numpy,abalkin/numpy,pbrod/numpy,pdebuyl/numpy,charris/numpy,rgommers/numpy,numpy/numpy,anntzer/numpy,seberg/numpy,pdebuyl/numpy,simongibbons/numpy,seberg/numpy,pdebuyl/numpy,grlee77/numpy,madphysicist/numpy,madphysicist/numpy,rgommers/numpy,pdebuyl/numpy,numpy/numpy,mattip/numpy,pbrod/numpy,mhvk/numpy,mhvk/numpy,jakirkham/numpy,grlee77/numpy,anntzer/numpy,madphysicist/numpy,simongibbons/numpy,numpy/numpy,grlee77/numpy,mattip/numpy,charris/numpy,pbrod/numpy,mattip/numpy,pbrod/numpy,simongibbons/numpy,rgommers/numpy,seberg/numpy,grlee77/numpy,jakirkham/numpy,jakirkham/numpy,mattip/numpy,grlee77/numpy,charris/numpy,abalkin/numpy,seberg/numpy,madphysicist/numpy,simongibbons/numpy,anntzer/numpy,abalkin/numpy,endolith/numpy,pbrod/numpy,endolith/numpy,rgommers/numpy,numpy/numpy,madphysicist/numpy,simongibbons/numpy,jakirkham/numpy,endolith/numpy,mhvk/numpy,endolith/numpy,mhvk/numpy,charris/numpy