Clarify documentation of IndexSeries

[h-mayorquin]

The documentation of IndexSeries looks confusing to me in the sense that the case of indexing an ImageSeries and the static images of an Images container seem to be mixed in the explanation:

nwb-schema/core/nwb.image.yaml

Lines 178 to 186 in d5a931d

	- neurodata_type_def: IndexSeries
	neurodata_type_inc: TimeSeries
	doc: Stores indices to image frames stored in an ImageSeries. The purpose of the
	IndexSeries is to allow a static image stack to be stored in an Images
	object, and the images in the stack to be referenced out-of-order. This can be for
	the display of individual images, or of movie segments (as a movie is simply a
	series of images). The data field stores the index of the frame in the referenced
	Images object, and the timestamps array indicates when that image
	was displayed.

Are both cases supported?
I can improve it to clarify that it can be used for both cases but I wanted to be sure.

[rly]

@h-mayorquin can you please create a PR to clarify the doc? We want to discourage the use of ImageSeries, and encourage the use of Images.