fmt: define what %p means (or doesn't mean) on interfaces

[bradfitz]

(split from #16230 (comment))

What does %p mean in the fmt package for interface values?

It seems to do something, but the documentation says nothing.

/cc @josharian @robpike

[josharian]

It's well-defined. The fmt package docs say: "Regardless of the verb, if an operand is an interface value, the internal concrete value is used, not the interface itself." This is a bug in vet, and I already have a fix in my tree for it as part of fixing #11041.

[gopherbot]

CL https://golang.org/cl/27127 mentions this issue.