DOC: update estimate_sigma docstring
DOC: update estimate_sigma docstring

rjeli/scikit-image,paalge/scikit-image,vighneshbirodkar/scikit-image,rjeli/scikit-image,rjeli/scikit-image,vighneshbirodkar/scikit-image,vighneshbirodkar/scikit-image,paalge/scikit-image,paalge/scikit-image