More updates describing FutureWarnings.
diff --git a/Doc/lib/libwarnings.tex b/Doc/lib/libwarnings.tex
index b0eb908..48efd68 100644
--- a/Doc/lib/libwarnings.tex
+++ b/Doc/lib/libwarnings.tex
@@ -68,6 +68,9 @@
 \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