At one point it was documented in the C preprocessor manual but it is no longer documented there.

Confirmed really it is documented but just under #import documentation.  It really should be under its own:
Another way to prevent a header file from being included more than once
is with the @samp{#pragma once} directive.  If @samp{#pragma once} is
seen when scanning a header file, that file will never be read again, no
matter what.

@samp{#pragma once} does not have the problems that @samp{#import} does,
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.