Document the behavior of creating empty subslice at end of slice

[sunfishcode]

Creating an empty subslice at the end of a slice succeeds: &slice[slice.len()..], however this doesn't appear to be documented in the slice documentation.

From some searching, I found efforts to document this behavior, including #32183, however this documentation doesn't seem to appear in slice's doc page or the index function's doc page.

The place I originally looked was on slice's get function.

This might be related to rustdoc omitting some documentation, however #31821 (comment) says to file specific issues, so here's a specific issue :-).