[BUG] doc/develop/logging.rst

Hello Simon,

I am trying to port the updated Sphinx build system from the Linux kernel to U-Boot so that we can use Sphinx 3.

With Sphinx 3 I get an error because the same C elements are described twice:

Warning, treated as error: doc/develop/logging:39: ./include/log.h:3: Duplicate C declaration, also defined at develop/logging:79.

I think the whole chapter "Logging API" can be moved to doc/api/ and all other kernel-doc:: should be replaced by cross-references.

Best regards

Heinrich