Patch #1504046: Add documentation for xml.etree.

/F wrote the text docs, Englebert Gruber massaged it to latex and I
did some more massaging to try and improve the consistency and
fix some name mismatches between the declaration and text.
diff --git a/Doc/lib/lib.tex b/Doc/lib/lib.tex
index 78efca8..b7e390f 100644
--- a/Doc/lib/lib.tex
+++ b/Doc/lib/lib.tex
@@ -171,6 +171,7 @@
 \input{xmlsaxhandler}
 \input{xmlsaxutils}
 \input{xmlsaxreader}
+\input{libetree}
 % \input{libxmllib}
 
 \input{fileformats}		% Miscellaneous file formats