Improve the description of CMS_get0_signers()

Improve the description of CMS_get0_signers()

Suggestion from Małgorzata Olszówka, they noted:

"The original wording suggests that it is required to execute
CMS_get0_signers() after CMS_verify(), while it is CMS_get0_signers()
that requires prior successful invocation of CMS_verify()."

OK tb@