update commonprefix doc to make sure user is aware it works
character-by-character and that that behavior may result in it returning
prefixes that are not valid paths
diff --git a/Doc/lib/libposixpath.tex b/Doc/lib/libposixpath.tex
index 4981ac0..66e2781 100644
--- a/Doc/lib/libposixpath.tex
+++ b/Doc/lib/libposixpath.tex
@@ -21,9 +21,11 @@
\end{funcdesc}
\begin{funcdesc}{commonprefix}{list}
-Return the longest string that is a prefix of all strings in
+Return the longest path prefix (taken character-by-character) that is a
+prefix of all paths in
\var{list}. If \var{list} is empty, return the empty string
-(\code{''}).
+(\code{''}). Note that this may return invalid paths because it works a
+character at a time.
\end{funcdesc}
\begin{funcdesc}{dirname}{path}