add info about slices to developer guide

[jasongrout]

Somewhere we should note the standard way to deal with getting the indices from a slice object is

range(*s.indices(size))

where s is our slice and size is the size of the object we are applying the slice to. See #4974 for lots of other information and discussion of various ways to do this, converging on the above simply standard python.

Component: documentation

Issue created by migration from https://trac.sagemath.org/ticket/4980