Clarified start parameter to Py_CompileString, documented

Py_eval_input, Py_file_input, and Py_single_input.

Problems reported by Aaron Brancotti <aaron@icona.it>.

Doc/api/api.tex

@@ -618,13 +618,32 @@ more detailed way with the interpreter.
                                                int start}
   Parse and compile the Python source code in \var{str}, returning the 
   resulting code object.  The start token is given by \var{start};
-  this can be used to constrain the code which can be compiled.  The
-  filename specified by \var{filename} is used to construct the code
-  object and may appear in tracebacks or \exception{SyntaxError}
-  exception messages.  This returns \NULL{} if the code cannot be
-  parsed or compiled.
+  this can be used to constrain the code which can be compiled and should
+  be \constant{Py_eval_input}, \constant{Py_file_input}, or
+  \constant{Py_single_input}.  The filename specified by
+  \var{filename} is used to construct the code object and may appear
+  in tracebacks or \exception{SyntaxError} exception messages.  This
+  returns \NULL{} if the code cannot be parsed or compiled.
 \end{cfuncdesc}
 
+\begin{cvardesc}{int}{Py_eval_input}
+  The start symbol from the Python grammar for isolated expressions;
+  for use with \cfunction{Py_CompileString()}.
+\end{cvardesc}
+
+\begin{cvardesc}{int}{Py_file_input}
+  The start symbol from the Python grammar for sequences of statements
+  as read from a file or other source; for use with
+  \cfunction{Py_CompileString()}.  This is the symbol to use when
+  compiling arbitrarily long Python source code.
+\end{cvardesc}
+
+\begin{cvardesc}{int}{Py_single_input}
+  The start symbol from the Python grammar for a single statement; for 
+  use with \cfunction{Py_CompileString()}.  This is the symbol used
+  for the interactive interpreter loop.
+\end{cvardesc}
+
 
 \chapter{Reference Counting \label{countingRefs}}