fmt: document existing behavior of nil receivers

fmt: document existing behavior of nil receivers

If a method called by fmt triggers a panic, the output usually says
so. However, there is heretofore undocumented special treatment for
a panic caused by formatting a nil value with an Error or String
method: the output is simply "<nil>". Document that behavior.

Change-Id: Id0f79dd0b3487f9d1c74a0856727bba5cc342be4
Reviewed-on: https://go-review.googlesource.com/6410
Reviewed-by: David Crawshaw <crawshaw@golang.org>