[DOC] Show `PTY.getpty` as an alias of `PTY.spawn`

`:nodoc:` directive does not work at method definition in C, and must
be at the implementation function.  That is, there is no way to make
one method visible and another method sharing the implementation
invisible at the same time.

ext/pty/pty.c

@@ -745,8 +745,13 @@ void
 Init_pty(void)
 {
     cPTY = rb_define_module("PTY");
-    /* :nodoc: */
-    rb_define_module_function(cPTY,"getpty",pty_getpty,-1);
+#if 1
+    rb_define_module_function(cPTY,"get""pty",pty_getpty,-1);
+#else  /* for RDoc */
+    /* show getpty as an alias of spawn */
+    VALUE sPTY = rb_singleton_class(cPTY);
+    rb_define_alias(sPTY, "getpty", "spawn");
+#endif
     rb_define_module_function(cPTY,"spawn",pty_getpty,-1);
     rb_define_singleton_method(cPTY,"check",pty_check,-1);
     rb_define_singleton_method(cPTY,"open",pty_open,0);