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

src/test/ui/span/issue-81800.rs

@@ -0,0 +1,2 @@
+fn x˂- //~ ERROR: unknown start of token
+       //~^ ERROR: expected one of `#`, `>`, `const`, identifier, or lifetime, found `-`

src/test/ui/span/issue-81800.stderr

@@ -0,0 +1,19 @@
+error: unknown start of token: \u{2c2}
+  --> $DIR/issue-81800.rs:1:5
+   |
+LL | fn x˂-
+   |     ^
+   |
+help: Unicode character '˂' (Modifier Letter Left Arrowhead) looks like '<' (Less-Than Sign), but it is not
+   |
+LL | fn x<-
+   |     ^
+
+error: expected one of `#`, `>`, `const`, identifier, or lifetime, found `-`
+  --> $DIR/issue-81800.rs:1:6
+   |
+LL | fn x˂-
+   |      ^ expected one of `#`, `>`, `const`, identifier, or lifetime
+
+error: aborting due to 2 previous errors
+