cmd/compile: document -l in godoc

Fixes #15607.

Change-Id: I3e68ad00ebe72027d064238d4e77f1ad6a52f533
Reviewed-on: https://go-review.googlesource.com/22940Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/doc.go

@@ -60,6 +60,8 @@ Flags:
 	-installsuffix suffix
 		Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
 		instead of $GOROOT/pkg/$GOOS_$GOARCH.
+	-l
+		Disable inlining.
 	-largemodel
 		Generate code that assumes a large memory model.
 	-linkobj file