clarify "why doesn't my shell script work"

winsup/doc/how-using.texinfo

@@ -233,17 +233,19 @@ bash$ mkdir -p path/to/new/dir
 @subsection Why doesn't my shell script work?
 
 There are two basic problems you might run into.  One is the fact that
-/bin/sh is really ash, and is missing some features you might expect in
-/bin/sh.  For example:
+/bin/sh is really ash, and is missing some features you might expect
+in /bin/sh, particularly if you are used to /bin/sh actually being
+bash (Linux) or ksh (Tru64).  For example:
 
 @itemize bullet
 @item No job control
 @item No getopts
+@item No let
 @item No functions exported
 @item Must use `.' instead of `source' (true of sh and ksh too, not just ash)
 @end itemize
 
-Or it could be a permission problem, and Cygwin doesn't understand that
+Or, it could be a permission problem, and Cygwin doesn't understand that
 your script is executable.  Because @samp{chmod} may not work (see FAQ
 entry above), Cygwin must read the contents of files to determine if
 they are executable.  If your script does not start with