Add abi.export_name.no_mangle

Specifies the interaction with export_name and no_mangle. This is
essentially a duplicate of abi.no_mangle.export_name, but I think it is
worthwile to say it in both places.

Author: ehuss

src/abi.md

@@ -167,6 +167,9 @@ r[abi.export_name.edition2024]
 > [!EDITION-2024]
 > Before the 2024 edition it is allowed to use the `export_name` attribute without the `unsafe` qualification.
 
+r[abi.export_name.no_mangle]
+If `export_name` is used with [`no_mangle`][abi.no_mangle], then the `export_name` is used instead.
+
 [attribute]: attributes.md
 [extern functions]: items/functions.md#extern-function-qualifier
 [external blocks]: items/external-blocks.md