Add abi.no_mangle.ascii-only

This is a restriction that wasn't previously documented.

Author: ehuss

src/abi.md

@@ -113,6 +113,9 @@ r[abi.no_mangle.edition2024]
 r[abi.no_mangle.publicly-exported]
 In addition to disabling name mangling, the item will be publicly exported from the produced library or object file, similar to the [`used` attribute](#the-used-attribute).
 
+r[abi.no_mangle.ascii-only]
+`no_mangle` may only be used on items with a name that only contains ASCII characters.
+
 r[abi.link_section]
 ## The `link_section` attribute