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/15952Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/doc.go

@@ -138,8 +138,8 @@
 	formatting considerations apply for operands that implement
 	certain interfaces. In order of application:
 
-	1. If the operand is a reflect.Value, the concrete value it
-	holds is printed as if it was the operand.
+	1. If the operand is a reflect.Value, the operand is replaced by the
+	concrete value that it holds, and printing continues with the next rule.
 
 	2. If an operand implements the Formatter interface, it will
 	be invoked. Formatter provides fine control of formatting.