cmd/compile: rename retsigerr to sigerr

retsigerr was used to create error message for both wrong function
arguments and return arguments, so change its name to sigerr to reflect
that.

While at it, also add documentation for the wrong function arguments
case.

Passes toolstash-check.

Change-Id: I740c717ad38d4afab9e8c20f2e94579c8bca67ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/255240
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/typecheck.go

diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index faa13d7..cbfaa30 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/src/cmd/compile/internal/gc/typecheck.go
@@ -2711,7 +2711,7 @@
 			return ""
 		}
 	}
-	return fmt.Sprintf("\n\thave %s\n\twant %v", nl.retsigerr(isddd), tstruct)
+	return fmt.Sprintf("\n\thave %s\n\twant %v", nl.sigerr(isddd), tstruct)
 }
 
 // sigrepr is a type's representation to the outside world,
@@ -2735,9 +2735,8 @@
 	return t.String()
 }
 
-// retsigerr returns the signature of the types
-// at the respective return call site of a function.
-func (nl Nodes) retsigerr(isddd bool) string {
+// sigerr returns the signature of the types at the call or return.
+func (nl Nodes) sigerr(isddd bool) string {
 	if nl.Len() < 1 {
 		return "()"
 	}