docs: fix an omission in the rtapi_shmem manpage

Make explicit an important promise made by rtapi_shmem_new(): When a
block of memory is first allocated, it is initialized to all zero bytes.

On Uspace rtapi_shmem_new() calls shmget(), which zeros the memory when
creating a new shared memory block:

    https://man7.org/linux/man-pages/man2/shmget.2.html#DESCRIPTION

On RTAI rtapi_shmem_new() calls rtai_kmalloc() and rtai_malloc(), which
both end up calling _rt_shmem_new().  The documentation does not say
anything about initialization of freshly allocated memory buffers, but
Paolo Mantegazza (one of the main RTAI developers) said that RTAI *does*
zeros shmem buffers, and pointed to this code:

    https://mail.rtai.org/pipermail/rtai/2022-January/028282.html
    http://cvs.savannah.nongnu.org/viewvc/rtai/magma/base/ipc/shm/shm.c?view=markup#l69

Author: SebKuzminsky

docs/man/man3/rtapi_shmem.3rtapi

@@ -35,8 +35,8 @@ wishing to access the same memory must use the same key.
 \fImodule_id\fR is the ID of the module that is making the call (see
 rtapi_init).  The block will be at least \fIsize\fR bytes, and may
 be rounded up.  Allocating many small blocks may be very wasteful.
-When a particular block is allocated for the first time, the first
-4 bytes are zeroed.  Subsequent allocations of the same block
+When a particular block is allocated for the first time, the contents
+are zeroed.  Subsequent allocations of the same block
 by other modules or processes will not touch the contents of the
 block.  Applications can use those bytes to see if they need to 
 initialize the block, or if another module already did so.