DOC: clarify range of `a` parameter fpr johnsonsu/johnsonsb (#13444)
DOC: clarify range of `a` parameter fpr johnsonsu/johnsonsb (#13444)

Closes gh-13353 
andyfaff/scipy,matthew-brett/scipy,endolith/scipy,anntzer/scipy,rgommers/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,ilayn/scipy,rgommers/scipy,e-q/scipy,WarrenWeckesser/scipy,mdhaber/scipy,matthew-brett/scipy,e-q/scipy,zerothi/scipy,e-q/scipy,zerothi/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,ilayn/scipy,ilayn/scipy,endolith/scipy,grlee77/scipy,mdhaber/scipy,grlee77/scipy,scipy/scipy,vigna/scipy,matthew-brett/scipy,rgommers/scipy,andyfaff/scipy,perimosocordiae/scipy,tylerjereddy/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,scipy/scipy,tylerjereddy/scipy,scipy/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,ilayn/scipy,anntzer/scipy,vigna/scipy,ilayn/scipy,vigna/scipy,Stefan-Endres/scipy,andyfaff/scipy,mdhaber/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,endolith/scipy,zerothi/scipy,vigna/scipy,scipy/scipy,endolith/scipy,perimosocordiae/scipy,andyfaff/scipy,WarrenWeckesser/scipy,grlee77/scipy,zerothi/scipy,mdhaber/scipy,grlee77/scipy,e-q/scipy,matthew-brett/scipy,perimosocordiae/scipy,rgommers/scipy,e-q/scipy,WarrenWeckesser/scipy,anntzer/scipy,ilayn/scipy,vigna/scipy,Stefan-Endres/scipy,matthew-brett/scipy,Eric89GXL/scipy,endolith/scipy,tylerjereddy/scipy,endolith/scipy,scipy/scipy,andyfaff/scipy,zerothi/scipy,grlee77/scipy,anntzer/scipy,zerothi/scipy,rgommers/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,anntzer/scipy,mdhaber/scipy,scipy/scipy,anntzer/scipy,mdhaber/scipy,tylerjereddy/scipy,Eric89GXL/scipy,tylerjereddy/scipy,andyfaff/scipy,Eric89GXL/scipy