documentation needs update

[drighelli]

Apparently, our documentation suggests that our class is only for Spatially resolved Transcriptomics data as also referred in our README SpatialExperiment is an R/Bioconductor S4 class for storing data from spatially resolved transcriptomics (ST) experiments.

It would be better to update it to let people understand that our class can accept any type of Single Cell Spatial -omics data, as SingleCellExperiment does.