Rollup merge of #132830 - wr7:substr_range_documentation, r=tgross35

Rename `elem_offset` to `element_offset`

Tracking issue: #126769

Renames `slice::elem_offset` to `slice::element_offset` and improves the documentation of it and its related methods.

The current documentation can be misinterpreted (as explained [here](rust-lang/rust#126769 (comment))).

Author: jhpratt