add docstring for `fusiontensor` #255

Job	Run time
Julia lts - ubuntu-latest - pull_request	8m 8s
Julia 1 - windows-latest - pull_request	14m 47s
Julia lts - macOS-latest - pull_request	8m 0s
Julia 1 - macOS-latest - pull_request	10m 28s
Julia 1 - ubuntu-latest - pull_request	11m 52s
Julia lts - windows-latest - pull_request	9m 54s
	1h 3m 9s

Provide feedback