Fix some trivial typos. Add a comment to a C file.

system/zmodem/README.txt

@@ -34,7 +34,7 @@ Buffering Notes
   Software Flow Control
   ---------------------
   The ZModem protocol has XON/XOFF flow control built into it.  The protocol
-  requires that XON or XOFF be placed at certain parts of messages.  If
+  permits XON or XOFF characters placed at certain parts of messages.  If
   software flow control is enabled on the receiving end it will consume the
   XONs and XOFFs.  Otherwise they will be ignored in the data by the ZModem
   logic.
@@ -45,7 +45,7 @@ Buffering Notes
   The XON/XOFF controls built into ZModem could be used if you enabled
   software flow control in the host.  But that would only work in one
   direction:  If would prevent the host from overrunning the the target Rx
-  buffing.  So you should be able to do host-to-target software flow
+  buffering.  So you should be able to do host-to-target software flow
   control.  But there would still be no target-to-host flow control.  That
   might not be an issue because the host is usually so much faster than
   that target.
@@ -53,7 +53,7 @@ Buffering Notes
   RX Buffer Size
   --------------
   The ZModem protocol supports a message that informs the file sender of
-  the maximum size of dat that you can buffer (ZRINIT).  However, my
+  the maximum size of data that you can buffer (ZRINIT).  However, my
   experience is that the Linux sz ignores this setting and always sends file
   data at the maximum size (1024) no matter what size of buffer you report.
   That is unfortunate because that, combined with the possibilities of data

system/zmodem/zm_send.c

@@ -1052,6 +1052,8 @@ static int zms_sendpacket(FAR struct zm_state_s *pzm)
         }
       else
         {
+          /* REVISIT: Why complemented? */
+
           crc = ~crc;
           for (i = 0; i < 4; i++, crc >>= 8)
             {