lib: rsa: add documentation to padding_pss_verify to document limitations

lib: rsa: add documentation to padding_pss_verify to document limitations

padding_pss_verify only works with the default pss salt setting of -2
(length to be automatically determined based on the PSS block structure)
not -1 (salt length set to the maximum permissible value), which makes
verifications of signatures with that saltlen fail.

Until this gets implemented at least document this behaviour.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>