runtime/debug: redirect in docs to runtime.Stack

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now that runtime.Stack exists we should encourage its use over runtime/debug's functions.
(Or perhaps runtime/debug.Stack should become a wrapper around runtime.Stack that takes
care of the allocation for you. But probably not.)

See also issue #3845.

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Comment 1:

Labels changed: added go1.1.

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Comment 2:

Labels changed: added size-s.

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Comment 3:

This issue was closed by revision e49a183.

Status changed to Fixed.