More updates describing FutureWarnings.

Doc/doc/doc.tex

@@ -1306,6 +1306,9 @@ This \UNIX\ is also followed by a space.
           features.}
   \lineii{RuntimeWarning}
          {Base category for warnings about dubious runtime features.}
+  \lineii{FutureWarning}
+	 {Base category for warnings about constructs that will change
+	 semantically in the future.}
 \end{tableii}
 \end{verbatim}
 

Doc/lib/libexcs.tex

@@ -362,7 +362,7 @@ Raised when an \keyword{assert} statement fails.
 \setindexsubitem{(built-in warning)}
 
 The following exceptions are used as warning categories; see the
-\module{warnings} module for more information.
+\refmodule{warnings} module for more information.
 
 \begin{excdesc}{Warning}
 Base class for warning categories.
@@ -388,6 +388,11 @@ Base class for warnings about dubious syntax
 Base class for warnings about dubious runtime behavior.
 \end{excdesc}
 
+\begin{excdesc}{FutureWarning}
+Base class for warnings about constructs that will change semantically
+in the future.
+\end{excdesc}
+
 The class hierarchy for built-in exceptions is:
 
 \begin{verbatim}
@@ -431,4 +436,5 @@ The class hierarchy for built-in exceptions is:
 	  +-- SyntaxWarning
 	  +-- OverflowWarning
 	  +-- RuntimeWarning
+	  +-- FutureWarning
 \end{verbatim}

Doc/lib/libwarnings.tex

@@ -68,6 +68,9 @@ syntactic features.}
 \lineii{RuntimeWarning}{Base category for warnings about dubious
 runtime features.}
 
+\lineii{FutureWarning}{Base category for warnings about constructs
+that will change semantically in the future.}
+
 \end{tableii}
 
 While these are technically built-in exceptions, they are documented