libalpm: edit docs for check_pgp_signature

The old wording implied that the function would return -1 if the
signature was invalid (alpm_sigresult_t->validity). When really it means
if a correctly formatted signature exists at all.

lib/libalpm/alpm.h

@@ -481,7 +481,7 @@ typedef struct _alpm_siglist_t {
  * Check the PGP signature for the given package file.
  * @param pkg the package to check
  * @param siglist a pointer to storage for signature results
- * @return 0 if valid, -1 if an error occurred or signature is invalid
+ * @return 0 on success, -1 if an error occurred or signature is missing
  */
 int alpm_pkg_check_pgp_signature(alpm_pkg_t *pkg, alpm_siglist_t *siglist);
 
@@ -489,7 +489,7 @@ int alpm_pkg_check_pgp_signature(alpm_pkg_t *pkg, alpm_siglist_t *siglist);
  * Check the PGP signature for the given database.
  * @param db the database to check
  * @param siglist a pointer to storage for signature results
- * @return 0 if valid, -1 if an error occurred or signature is invalid
+ * @return 0 on success, -1 if an error occurred or signature is missing
  */
 int alpm_db_check_pgp_signature(alpm_db_t *db, alpm_siglist_t *siglist);