Add documents from envos/LISPCORE/internal/docs (#812)

docs/internal/RETCALL.MAIL

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+*start*
+00530 00024 US 
+From: masinter.PA
+Date: 17-Jul-86 22:42:29 PDT
+Subject: Re: [STEVER%OZ.AI.MIT.EDU@XX.LCS.MIT.EDU: Tail Recursion in Common Lisp ???]
+In-reply-to: Pavel's message of 17 Jul 86 20:57 PDT
+To: Pavel
+cc: TamarinCore^
+
+for the curious, I did an experiment with tail recursive call. There's an "opcode" called RETCALL, complete with UFN which fakes it using the standard awful hacks.
+
+The simplest spec is that
+
+RETCALL <n> <name>
+
+works like
+
+FNX <n> <name> 
+
+except that it throws away the current frame first.