uthread: doc: fix inline documentation

uthread: doc: fix inline documentation

Fix Sphinx warnings:

 $ make htmldocs
 [...]
 ./include/uthread.h:56: warning: cannot understand function prototype: 'enum uthread_mutex_state '
 ./include/uthread.h:64: warning: cannot understand function prototype: 'struct uthread_mutex '
 ./include/uthread.h:56: warning: cannot understand function prototype: 'enum uthread_mutex_state '
 ./include/uthread.h:64: warning: cannot understand function prototype: 'struct uthread_mutex '

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reported-by: Tom Rini <trini@konsulko.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>