[DOC PATCH] av.c - clearify that av_shift returns &PL_sv_undef if array is empty

Hi,

The docs for av_pop says that &PL_sv_undef is returned if the array is  
empty and since av_shift does the same I think the docs should state  
that too.

Attached is a tiny doc patch for that.

/Claes

• [DOC PATCH] av.c - clearify that av_shift returns &PL_sv_undef if array is empty by Claes Jakobsson
• Re: [DOC PATCH] av.c - clearify that av_shift returns &PL_sv_undef if array is empty by Rafael Garcia-Suarez