item search: clarify antimeridian handling of bbox

[soxofaan]

The readme of item-search (https://github.com/radiantearth/stac-api-spec/blob/release/v1.0.0/item-search/README.md) does mention bbox as core query parameter, but just describes it as "bounding box". It does not mention how antimeridian cases should be handled.

The openapi.yaml file however does have some clarification:

stac-api-spec/item-search/openapi.yaml

Lines 153 to 157 in 604ade6

	For WGS 84 longitude/latitude the values are in most cases the sequence
	of minimum longitude, minimum latitude, maximum longitude and maximum
	latitude. However, in cases where the box spans the antimeridian the
	first value (west-most box edge) is larger than the third value
	(east-most box edge).

I think it's useful to mention that detail in the readme too