doc: mark signing the binary is macOS and Windows only in single-executable docs

Author: meixg

doc/api/single-executable-applications.md

@@ -33,7 +33,7 @@ tool, [postject][]:
    $ cp $(command -v node) hello
    ```
 
-3. Remove the signature of the binary:
+3. Remove the signature of the binary (macOS and Windows only):
 
    * On macOS:
 
@@ -78,7 +78,7 @@ tool, [postject][]:
          --macho-segment-name NODE_JS
      ```
 
-5. Sign the binary:
+5. Sign the binary (macOS and Windows only):
 
    * On macOS: