syscall: improve NewCallback documentation and panic message

Fixes #26138

Change-Id: If77b2839bccc600223735df42438a19131cd051c
GitHub-Last-Rev: 64ceaea9f1cb7afb3d552dce0223b818bac1faf9
GitHub-Pull-Request: golang/go#26617
Reviewed-on: https://go-review.googlesource.com/126035
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 6 insertions, 6 deletions

diff --git a/src/syscall/syscall_windows.go b/src/syscall/syscall_windows.go
index 5cfdb76e2b..b234f3d67d 100644
--- a/src/syscall/syscall_windows.go
+++ b/src/syscall/syscall_windows.go
@@ -120,16 +120,16 @@ func (e Errno) Timeout() bool {
 // Implemented in runtime/syscall_windows.go.
 func compileCallback(fn interface{}, cleanstack bool) uintptr
 
-// Converts a Go function to a function pointer conforming
-// to the stdcall calling convention. This is useful when
-// interoperating with Windows code requiring callbacks.
+// NewCallback converts a Go function to a function pointer conforming to the stdcall calling convention.
+// This is useful when interoperating with Windows code requiring callbacks.
+// The argument is expected to be a function with with one uintptr-sized result. The function must not have arguments with size larger than the size of uintptr.
 func NewCallback(fn interface{}) uintptr {
 	return compileCallback(fn, true)
 }
 
-// Converts a Go function to a function pointer conforming
-// to the cdecl calling convention. This is useful when
-// interoperating with Windows code requiring callbacks.
+// NewCallbackCDecl converts a Go function to a function pointer conforming to the cdecl calling convention.
+// This is useful when interoperating with Windows code requiring callbacks.
+// The argument is expected to be a function with with one uintptr-sized result. The function must not have arguments with size larger than the size of uintptr.
 func NewCallbackCDecl(fn interface{}) uintptr {
 	return compileCallback(fn, false)
 }