runtime/pprof: restore articles in type doc comments

Reverts part of CL 6460082.

If a doc comment describes a type by explaining the
meaning of one instance of the type, a leading article
is fine and makes the text less awkward.

Compare:
// A dog is a kind of animal.
// Dog is a kind of animal.

R=golang-dev, dsymonds, dvyukov, r
CC=golang-dev
https://golang.org/cl/6494066

src/pkg/runtime/pprof/pprof.go

@@ -23,7 +23,7 @@ import (
 // BUG(rsc): A bug in the OS X Snow Leopard 64-bit kernel prevents
 // CPU profiling from giving accurate results on that system.
 
-// Profile is a collection of stack traces showing the call sequences
+// A Profile is a collection of stack traces showing the call sequences
 // that led to instances of a particular event, such as allocation.
 // Packages can create and maintain their own profiles; the most common
 // use is for tracking resources that must be explicitly closed, such as files
@@ -250,7 +250,7 @@ func (x stackProfile) Less(i, j int) bool {
 	return len(t) < len(u)
 }
 
-// countProfile is a set of stack traces to be printed as counts
+// A countProfile is a set of stack traces to be printed as counts
 // grouped by stack trace.  There are multiple implementations:
 // all that matters is that we can find out how many traces there are
 // and obtain each trace in turn.