cmd/compile: document that nosplit directive is unsafe

For #74478

Change-Id: I902e9a92cdacb5ad6dafa9896640f8196ba1d56a
Reviewed-on: https://go-review.googlesource.com/c/go/+/686115
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: ianlancetaylor

src/cmd/compile/doc.go

@@ -253,6 +253,9 @@ The //go:nosplit directive must be followed by a function declaration.
 It specifies that the function must omit its usual stack overflow check.
 This is most commonly used by low-level runtime code invoked
 at times when it is unsafe for the calling goroutine to be preempted.
+Using this directive outside of low-level runtime code is not safe,
+because it permits the nosplit function to overwrite the end of stack,
+leading to memory corruption and arbitrary program failure.
 
 # Linkname Directive