Improve documentation of `read.image`

[lambdamoses]

Specifically the documentation of the subset argument, which isn't very clear. I need to look into the source code to figure out what to put for subset when reading in only a subset of the image. Basically, according to the source code, subset should be a named list whose names are among "x", "y", "c", "z", and "t", corresponding to the dimensions of x, y, channel, z, and time. Each element of the list is integer indices of pixels to read in the dimension of interest just like when subsetting matrices, and if it's NULL then all in that dimension is read.

https://github.com/aoles/RBioFormats/blob/1e715b164cf412d7a7674df0681b81841e95279e/R/read.image.R#L74-L81