Mention that .pth files can't be used to override standard modules
(suggested by Jack Jansen)
diff --git a/Doc/inst/inst.tex b/Doc/inst/inst.tex
index f86fed3..f0ee44c 100644
--- a/Doc/inst/inst.tex
+++ b/Doc/inst/inst.tex
@@ -728,10 +728,14 @@
directory that's already on Python's path, usually to the
\file{.../site-packages/} directory. Path configuration files have an
extension of \file{.pth}, and each line must contain a single path
-that will be added to \code{sys.path}. Paths can be absolute or
-relative, in which case they're relative to the directory containing
-the \file{.pth} file. Any directories added to the search path will
-be scanned in turn for \file{.pth} files. See
+that will be appended to \code{sys.path}. (Because the new paths are
+appended to \code{sys.path}, modules in the added directories will not
+override standard modules. This means you can't use this mechanism
+for installing fixed versions of standard modules.)
+
+Paths can be absolute or relative, in which case they're relative to
+the directory containing the \file{.pth} file. Any directories added
+to the search path will be scanned in turn for \file{.pth} files. See
\citetitle[http://www.python.org/dev/doc/devel/lib/module-site.html]{the
documentation for the \module{site} module} for more information.