DOC: Add missing parameter description for keepdims in MaskedArray
DOC: Add missing parameter description for keepdims in MaskedArray

The methods max, min, ptp were missing the parameter
in the documentation.

WarrenWeckesser/numpy,madphysicist/numpy,mhvk/numpy,charris/numpy,madphysicist/numpy,grlee77/numpy,charris/numpy,WarrenWeckesser/numpy,grlee77/numpy,WarrenWeckesser/numpy,ahaldane/numpy,MSeifert04/numpy,MSeifert04/numpy,seberg/numpy,pbrod/numpy,pizzathief/numpy,rgommers/numpy,endolith/numpy,abalkin/numpy,anntzer/numpy,WarrenWeckesser/numpy,madphysicist/numpy,ahaldane/numpy,numpy/numpy,jakirkham/numpy,madphysicist/numpy,mattip/numpy,charris/numpy,pizzathief/numpy,pbrod/numpy,mhvk/numpy,seberg/numpy,pizzathief/numpy,mattip/numpy,simongibbons/numpy,seberg/numpy,endolith/numpy,jakirkham/numpy,endolith/numpy,grlee77/numpy,mattip/numpy,pbrod/numpy,anntzer/numpy,WarrenWeckesser/numpy,endolith/numpy,MSeifert04/numpy,rgommers/numpy,grlee77/numpy,simongibbons/numpy,simongibbons/numpy,anntzer/numpy,MSeifert04/numpy,ahaldane/numpy,anntzer/numpy,numpy/numpy,mhvk/numpy,mhvk/numpy,grlee77/numpy,jakirkham/numpy,pizzathief/numpy,jorisvandenbossche/numpy,rgommers/numpy,pbrod/numpy,ahaldane/numpy,pdebuyl/numpy,madphysicist/numpy,jakirkham/numpy,jorisvandenbossche/numpy,seberg/numpy,numpy/numpy,abalkin/numpy,jakirkham/numpy,rgommers/numpy,pdebuyl/numpy,ahaldane/numpy,pizzathief/numpy,simongibbons/numpy,jorisvandenbossche/numpy,mhvk/numpy,charris/numpy,numpy/numpy,jorisvandenbossche/numpy,pbrod/numpy,simongibbons/numpy,pdebuyl/numpy,mattip/numpy,jorisvandenbossche/numpy,abalkin/numpy,pdebuyl/numpy,MSeifert04/numpy