plan9, unix, windows: add paragraph breaks in docs

Currently the doc string forms one run-on sentence when viewed in
a browser. Add an additional newline so the text is presented in
a browser the same way it is in the source code.

Change-Id: If93e45c5aec9edd195c6bf36cb757c5146368507
Reviewed-on: https://go-review.googlesource.com/101284
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

plan9/syscall.go

diff --git a/plan9/syscall.go b/plan9/syscall.go
index 5046cfe..163254c 100644
--- a/plan9/syscall.go
+++ b/plan9/syscall.go
@@ -11,11 +11,14 @@
 // system, set $GOOS and $GOARCH to the desired system. For example, if
 // you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
 // to freebsd and $GOARCH to arm.
+//
 // The primary use of this package is inside other packages that provide a more
 // portable interface to the system, such as "os", "time" and "net".  Use
 // those packages rather than this one if you can.
+//
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
+//
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.ErrorString.

unix/syscall.go

diff --git a/unix/syscall.go b/unix/syscall.go
index 857d2a4..1469079 100644
--- a/unix/syscall.go
+++ b/unix/syscall.go
@@ -11,11 +11,14 @@
 // system, set $GOOS and $GOARCH to the desired system. For example, if
 // you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
 // to freebsd and $GOARCH to arm.
+//
 // The primary use of this package is inside other packages that provide a more
 // portable interface to the system, such as "os", "time" and "net".  Use
 // those packages rather than this one if you can.
+//
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
+//
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.Errno.

windows/syscall.go

diff --git a/windows/syscall.go b/windows/syscall.go
index b07bc23..af828a9 100644
--- a/windows/syscall.go
+++ b/windows/syscall.go
@@ -11,11 +11,14 @@
 // system, set $GOOS and $GOARCH to the desired system. For example, if
 // you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
 // to freebsd and $GOARCH to arm.
+//
 // The primary use of this package is inside other packages that provide a more
 // portable interface to the system, such as "os", "time" and "net".  Use
 // those packages rather than this one if you can.
+//
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
+//
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.Errno.