Update 'Return Direction'

Return-Direction.md

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+The way ARF handles return values from functions is different from other
+languages, and is made possible by its ability to define variables in-place. It
+is done using an arrow called a return direction:
+
+`file.write bytes -> bytesWritten:Size err`
+
+The advantage of this is a new variable can be defined without having to
+re-define and therefore shadow `err`.
+
+If a phrase is enclosed in brackets, the return direction arrow lies outside of
+them:
+
+`[file.write bytes] -> bytesWritten:Size err`
+
+Return direction cannot be used with phrases that are already being used as
+arguments. Its one of those things that, very elegantly, makes no sense
+conceptually or syntactically.