fmt: clarify reflect.Value printing

fmt: clarify reflect.Value printing

fmt docs say:

  If the operand is a reflect.Value, the concrete value it
  holds is printed as if it was the operand.

It implies recursive application of this rule, which is not the case.
Clarify the docs.

Change-Id: I019277c7c6439095bab83e5536aa06403638aa51
Reviewed-on: https://go-review.googlesource.com/15952
Reviewed-by: Ian Lance Taylor <iant@golang.org>