Add documentation for VariantWith.
Add documentation for VariantWith.

antidotcb/googletest,jlanecox/googletest,xnagireddy/googletest,gianricardo/googletest,antidotcb/googletest,jlanecox/googletest,fuchsia-mirror/third_party-googletest,xnagireddy/googletest,google/googletest,xnagireddy/googletest,google/googletest,google/googletest,gianricardo/googletest,empiredan/googletest,gianricardo/googletest,antidotcb/googletest,xnagireddy/googletest,empiredan/googletest,fuchsia-mirror/third_party-googletest,fuchsia-mirror/third_party-googletest,jlanecox/googletest,google/googletest,fuchsia-mirror/third_party-googletest,jlanecox/googletest,antidotcb/googletest,empiredan/googletest