Update hook_civicrm_aclWhereClause.md

Clarify that the ContactID refers to the user making the query - my reading had been that it was the contact ID of the contact that was being considered.

docs/hooks/hook_civicrm_aclWhereClause.md

@@ -26,7 +26,8 @@ query is performed. Those will join onto the
     select clause
 -   array $whereTables - (reference ) add the tables that are needed
     for the where clause
--   int $contactID - the contactID for whom the check is made
+-   int $contactID - the contactID for whom the check is made, i.e. the ContactID of
+    the user trying to access the contacts.
 -   string $where - the currrent where clause
 
 ## Returns
@@ -84,4 +85,4 @@ query is performed. Those will join onto the
         if ( ! empty( $clauses ) ) {
             $where .= ' AND (' . implode( ' OR ', $clauses ) . ')';
         }
-    }
\ No newline at end of file
+    }