[perl #113782] [PATCH variants] pp_sassign: explain the mix of left<=>right in the optree

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This is a bug report for perl from rurban <at> cpanel.net,
generated with the help of perlbug 1.39 running under perl 5.17.0.

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I will attach two variants of patches to fix the pp_sassign 
left<=>right mixup problem.

1. Explain in the ASSIGNOP and in pp_sassign that the two arguments
   for sassign are backwards, right means left hand side in the source,
   left means right hand side.

2. (Preferred variant) Fix left and right in pp_sassign to make it readable.
    lhs and rhs reflect now the parsed source not the position on the optree
    stack. This matches aassign and the terminology in the parser.

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Flags:
    category=core
    severity=medium
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