doc: document new restriction on cgo-generated method receicer types

Follow-up on CL 629715.

For #60725.

Change-Id: I1b980ad44f73550b633c74fc881c70255e7d8565
Reviewed-on: https://go-review.googlesource.com/c/go/+/630335
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Bypass: Robert Griesemer <[email protected]>

Author: griesemer

doc/next/5-toolchain.md

@@ -1,5 +1,11 @@
 ## Compiler {#compiler}
 
+<!-- go.dev/issue/60725, go.dev/issue/57926 -->
+The compiler already disallowed defining new methods with receiver types that were
+cgo-generated, but it was possible to circumvent that restriction via an alias type.
+Go 1.24 now always reports an error if a receiver denotes a cgo-generated type,
+whether directly or indirectly (through an alias type).
+
 ## Assembler {#assembler}
 
 ## Linker {#linker}