Fix two small errors in argument lists.

Doc/lib/libdecimal.tex

@@ -713,8 +713,8 @@ here.
   \constant{NaN}.
 \end{methoddesc}
 
-\begin{methoddesc}{sqrt}{}
-  Return the square root to full precision.
+\begin{methoddesc}{sqrt}{x}
+  Return the square root of \var{x} to full precision.
 \end{methoddesc}                    
 
 \begin{methoddesc}{subtract}{x, y}
@@ -734,7 +734,7 @@ here.
   or \constant{Rounded}.                                        
 \end{methoddesc} 
 
-\begin{methoddesc}{to_sci_string}{}
+\begin{methoddesc}{to_sci_string}{x}
   Converts a number to a string using scientific notation.
 \end{methoddesc}