With previous sed expression it was failing e.g. on runtime.guintptr,
giving much more after type definition:

	$ go doc -c -u runtime.muintptr | sed -n -e '/^type /,/^}/p'
	type muintptr uintptr
	    muintptr is a *m that is not tracked by the garbage collector.

	    Because we do free Ms, there are some additional constrains on muintptrs:

	    1. Never hold an muintptr locally across a safe point.

	    2. Any muintptr in the heap must be owned by the M itself so it can

	    ensure it is not in use when the last true *m is released.

	func (mp muintptr) ptr() *m
	func (mp *muintptr) set(m *m)

Fix it since we'll need to extract more single-line types for Go1.11 support.

To verify it works de-stub {g,p,m}uintptr.