sandbox: document parameters of os_realloc()

Avoid 'make htmldocs' build warnings:

    ./include/os.h:139: warning:
    Function parameter or member 'ptr' not described in 'os_realloc'
    ./include/os.h:139: warning:
    Function parameter or member 'length' not described in 'os_realloc'

Fixes: 14e46dfb ("sandbox: Add os_realloc()")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>