[docs] Use ``\boldsymbol{\Sigma}``, not ``\mathbf{\Sigma}``
[docs] Use ``\boldsymbol{\Sigma}``, not ``\mathbf{\Sigma}``

This prevents problems when using other fonts than the default for LaTeX
builds; or when switching to xelatex with the LaTeX default OpenType
font set-up using fontspec.

The problem shows in that latter case via such message in the LaTeX log:

Missing character: There is no ^^F in font [lmroman10-regular]:mapping=tex-text ;!

perimosocordiae/scipy,endolith/scipy,jor-/scipy,zerothi/scipy,WarrenWeckesser/scipy,zerothi/scipy,scipy/scipy,mdhaber/scipy,andyfaff/scipy,zerothi/scipy,tylerjereddy/scipy,zerothi/scipy,tylerjereddy/scipy,andyfaff/scipy,aeklant/scipy,Stefan-Endres/scipy,jor-/scipy,aeklant/scipy,person142/scipy,andyfaff/scipy,matthew-brett/scipy,endolith/scipy,jor-/scipy,jamestwebber/scipy,ilayn/scipy,gertingold/scipy,e-q/scipy,mdhaber/scipy,gfyoung/scipy,nmayorov/scipy,anntzer/scipy,scipy/scipy,lhilt/scipy,aarchiba/scipy,aarchiba/scipy,gertingold/scipy,mdhaber/scipy,WarrenWeckesser/scipy,lhilt/scipy,jor-/scipy,e-q/scipy,Eric89GXL/scipy,gfyoung/scipy,endolith/scipy,mdhaber/scipy,ilayn/scipy,perimosocordiae/scipy,andyfaff/scipy,gertingold/scipy,person142/scipy,scipy/scipy,Eric89GXL/scipy,rgommers/scipy,Stefan-Endres/scipy,e-q/scipy,endolith/scipy,gfyoung/scipy,Eric89GXL/scipy,matthew-brett/scipy,WarrenWeckesser/scipy,vigna/scipy,ilayn/scipy,matthew-brett/scipy,perimosocordiae/scipy,zerothi/scipy,anntzer/scipy,lhilt/scipy,pizzathief/scipy,jamestwebber/scipy,endolith/scipy,vigna/scipy,e-q/scipy,scipy/scipy,arokem/scipy,Eric89GXL/scipy,vigna/scipy,Stefan-Endres/scipy,gertingold/scipy,rgommers/scipy,jamestwebber/scipy,lhilt/scipy,gertingold/scipy,pizzathief/scipy,WarrenWeckesser/scipy,jamestwebber/scipy,e-q/scipy,anntzer/scipy,grlee77/scipy,zerothi/scipy,perimosocordiae/scipy,grlee77/scipy,tylerjereddy/scipy,gfyoung/scipy,perimosocordiae/scipy,nmayorov/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,tylerjereddy/scipy,matthew-brett/scipy,rgommers/scipy,rgommers/scipy,grlee77/scipy,Eric89GXL/scipy,scipy/scipy,ilayn/scipy,scipy/scipy,pizzathief/scipy,ilayn/scipy,anntzer/scipy,gfyoung/scipy,anntzer/scipy,matthew-brett/scipy,vigna/scipy,ilayn/scipy,anntzer/scipy,person142/scipy,person142/scipy,grlee77/scipy,Stefan-Endres/scipy,aeklant/scipy,person142/scipy,arokem/scipy,arokem/scipy,mdhaber/scipy,mdhaber/scipy,andyfaff/scipy,arokem/scipy,pizzathief/scipy,grlee77/scipy,nmayorov/scipy,aeklant/scipy,arokem/scipy,endolith/scipy,aeklant/scipy,nmayorov/scipy,Eric89GXL/scipy,tylerjereddy/scipy,aarchiba/scipy,lhilt/scipy,jamestwebber/scipy,aarchiba/scipy,Stefan-Endres/scipy,WarrenWeckesser/scipy,aarchiba/scipy,rgommers/scipy,vigna/scipy,Stefan-Endres/scipy,andyfaff/scipy,pizzathief/scipy,jor-/scipy,nmayorov/scipy