doc: strel decomposition is often performed automatically (bug #45447)
* @strel/strel.m: remove snippet showing that speed increase when using a
decomposed strel. Because imdilate and imerode automatically perform
decomposition, and because strel() can guess rectangular SE, the performance
increase does not exist. It may come to exist for other shapes.
* @strel/getsequence.m: explain why SE decomposition may lead to performance
gains and why it may not visible.