doc: mark signing the binary is macOS and Windows only in SEA

PR-URL: nodejs#47722
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Qingyu Deng <i@ayase-lab.com>

doc/api/single-executable-applications.md

@@ -66,7 +66,7 @@ tool, [postject][]:
 
    The `.exe` extension is necessary.
 
-5. Remove the signature of the binary:
+5. Remove the signature of the binary (macOS and Windows only):
 
    * On macOS:
 
@@ -118,7 +118,7 @@ tool, [postject][]:
          --macho-segment-name NODE_SEA
      ```
 
-7. Sign the binary:
+7. Sign the binary (macOS and Windows only):
 
    * On macOS: