Rollup merge of #141666 - lolbinarycat:rustdoc-source_span_for_markdown_range-bug-141665, r=GuillaumeGomez

source_span_for_markdown_range: fix utf8 violation

it is non-trivial to reproduce this bug through rustdoc, which uses this function less than clippy, so the regression test was added as a unit test instead of an integration test.

fixes rust-lang/rust#141665

r? ``@GuillaumeGomez``

Author: GuillaumeGomez