improve documentation for 'var T return values'; refs #7373

doc/manual/var_t_return.rst

@@ -0,0 +1,20 @@
+Memory safety for returning by ``var T`` is ensured by a simple borrowing
+rule: If ``result`` does not refer to a location pointing to the heap
+(that is in ``result = X`` the ``X`` involves a ``ptr`` or ``ref`` access)
+then it has to be deviated by the routine's first parameter:
+
+.. code-block:: nim
+  proc forward[T](x: var T): var T =
+    result = x # ok, deviated from the first parameter.
+
+  proc p(param: var int): var int =
+    var x: int
+    # we know 'forward' provides a view into the location deviated by
+    # its first argument 'x'.
+    result = forward(x) # Error: location is derived from ``x``
+                        # which is not p's first parameter and lives
+                        # on the stack.
+
+In other words, the lifetime of what ``result`` points to is attached to the
+lifetime of the first parameter and that is enough knowledge to verify
+memory safety at the callsite.