Rollup merge of #81860 - osa1:issue81800, r=estebank

Fix SourceMap::start_point

`start_point` needs to return the *first* character's span, but it would
previously call `find_width_of_character_at_span` which returns the span
of the *last* character. The implementation is now fixed.

Other changes:

- Docs for start_point, end_point, find_width_of_character_at_span
  updated

- Minor simplification in find_width_of_character_at_span code

Fixes #81800

diff --cc compiler/rustc_span/src/source_map.rs
Simple merge