docs: pstore-blk.rst: fix kernel-doc tags

There is currently a problem with kernel-doc tags from blk.c:

	.../Documentation/admin-guide/pstore-blk:239: ./fs/pstore/blk.c:175: WARNING: Duplicate C declaration, also defined in 'admin-guide/pstore-blk'.
	Declaration is 'register_pstore_device'.
	.../Documentation/admin-guide/pstore-blk:239: ./fs/pstore/blk.c:432: WARNING: Duplicate C declaration, also defined in 'admin-guide/pstore-blk'.
	Declaration is 'register_pstore_blk'.
	.../Documentation/admin-guide/pstore-blk:242: ./include/linux/pstore_blk.h:43: WARNING: Duplicate C declaration, also defined in 'admin-guide/pstore-blk'.
	Declaration is 'pstore_device_info'.

Basically, the internal parts is shown with :export:, instead
of :internal:. Yet, there are some other exported docs that
aren't at the document, because they lack :identifiers:.

So, instead, let's just use :export: at the kAPI part of
the documentation.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>