crypto: doc - optimize compilation

The :functions: definition allows the specification of multiple
function references which prevents parsing the header file multiple
times.

Reported-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

crypto: doc - add KPP documentation

Add the KPP API documentation to the kernel crypto API Sphinx
documentation. This addition includes the documentation of the
ECDH and DH helpers which are needed to create the approrpiate input
data for the crypto_kpp_set_secret function.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>