doc/C API: Add tst_tmpdir.h

Header created in the previous commit.

Link: https://lore.kernel.org/ltp/20240712081359.116227-3-pvorel@suse.cz/
Reviewed-by: Li Wang <liwang@redhat.com>
Reviewed-by: Avinesh Kumar <akumar@suse.de>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

doc/developers/api_c_tests.rst

@@ -37,3 +37,7 @@ Capabilities
 libltpswap
 ----------
 .. kernel-doc:: ../../include/libswap.h
+
+Temporary directory
+-------------------
+.. kernel-doc:: ../../include/tst_tmpdir.h