Merge pull request #484 from Don-Ward/ready-doc

Clarify the behaviour of ready() in the Language Reference.

doc/book/langref.tex

@@ -3091,10 +3091,10 @@ \subsection*{Built-in functions}
 
 \noindent
 \index{ready(f,i)}\texttt{ready(f,i)} reads up to \texttt{i} characters
-from file \texttt{f}. It returns immediately with available data and
+from file \texttt{f}. If \texttt{i} is positive it returns immediately with available data or
 fails if no data is available. If \texttt{i} is 0, \texttt{ready()}
-returns all available input. It is not currently implemented for window
-values.
+returns all available input or an empty string if no data is available.
+It is not currently implemented for window values.
 
 \bigskip\hrule\vspace{0.1cm}
 \index{real}