Update doc/user/project/repository/gpg_signed_commits/index.md with clarifying...

Update doc/user/project/repository/gpg_signed_commits/index.md with clarifying gpg command for generating keys on other operating systems

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36923.

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6492.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/user/project/repository/gpg_signed_commits/index.md

@@ -56,6 +56,8 @@ started:
     gpg --full-gen-key
     ```
     
+_NOTE: In some cases like Gpg4win on Windows and other Mac OS versions the command here may be ` gpg --gen-key`_
+
     This will spawn a series of questions.
 
 1. The first question is which algorithm can be used.  Select the kind you want