Clarify how hook_compressed determines how to open a file.

Doc/lib/libfileinput.tex

@@ -165,8 +165,9 @@ filtering is disabled when standard input is read.
 The two following opening hooks are provided by this module:
 
 \begin{funcdesc}{hook_compressed}{filename, mode}
-  Transparently opens files compressed with gzip and bzip2 using
-  the \module{gzip} and \module{bz2} modules.
+  Transparently opens files compressed with gzip and bzip2 (recognized
+  by the extensions \code{'.gz'} and \code{'.bz2'}) using the \module{gzip}
+  and \module{bz2} modules as well as normal files.
 
   Usage example: 
   \samp{fi = fileinput.FileInput(openhook=fileinput.hook_compressed)}