docs-rst: don't ignore internal functions for jbd2 docs

Those functions are currently ignored, causing references at
the documentation to be lost. Don't ignore it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/filesystems/index.rst

@@ -287,7 +287,6 @@ Transasction Level
 ~~~~~~~~~~~~~~~~~~
 
 .. kernel-doc:: fs/jbd2/transaction.c
-   :export:
 
 See also
 --------