Document ImportWarning

Doc/lib/libexcs.tex

@@ -451,6 +451,11 @@ Base class for warnings about constructs that will change semantically
 in the future.
 \end{excdesc}
 
+\begin{excdesc}{ImportWarning}
+Base class for warnings about probable mistakes in module imports.
+\versionadded{2.5}
+\end{excdesc}
+
 The class hierarchy for built-in exceptions is:
 
 \verbatiminput{../../Lib/test/exception_hierarchy.txt}