cmd/preprofile, runtime/coverage: add package comment

As https://go.dev/doc/comment#package says, every package
should have a package comment. Command cmd/preprofile had
one, it was just not being recognized due to a blank line.

For #51430.
For #58102.

Change-Id: I73e31158e0f244f6453728ab68c5c8da4cfb38b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/613375
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/preprofile/main.go

@@ -9,9 +9,6 @@
 // Usage:
 //
 //	go tool preprofile [-v] [-o output] -i input
-//
-//
-
 package main
 
 import (

src/runtime/coverage/coverage.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package coverage contains APIs for writing coverage profile data at runtime
+// from long-running and/or server programs that do not terminate via [os.Exit].
 package coverage
 
 import (