Update SIGNING_KEY_ID description in codesigning.md

Clarified the definition of SIGNING_KEY_ID with additional details on obtaining it.

Author: chennes

technical/codesigning.md

@@ -61,7 +61,7 @@ The complete process is:
     * `DEVELOPER_TEAM_ID` -- Either the FPA's team ID (visible in your Apple developer account), or your own ID team ID from Apple
     * `KEYCHAIN_PASSWORD` -- An arbitrary password, used on the CI system as the password for the keychain. Anything you want.
     * `P12_PASSWORD` -- The password you set when exporting the certificate to the `*.p12` file.
-    * `SIGNING_KEY_ID` -- The ID of the certificate associated with `BUILD_PROVISION_PROFILE_BASE64`
+    * `SIGNING_KEY_ID` -- The ID of the certificate associated with `BUILD_PROVISION_PROFILE_BASE64`. Obtained by running `security find-identity -v -p codesigning` on a machine with the key installed. It is the SHA hash of the selected key in the displayed list.
 16. Run the "Weekly Build" GitHub action to generate the signed macOS weeklies based on these settings.
 
 ## Windows