_content: link to definition of "first sentence"

Fixes golang/go#59206

Change-Id: Ib3b92614acbceb5452c2a5d08623cc2e6cf5b182
Reviewed-on: https://go-review.googlesource.com/c/website/+/496495
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/blog/godoc.md

@@ -74,8 +74,9 @@ the package comment is placed in its own file,
 [doc.go](/src/pkg/encoding/gob/doc.go),
 which contains only those comments and a package clause.
 
-When writing package comments of any size,
-keep in mind that their first sentence will appear in godoc's [package list](/pkg/).
+When writing a package comment of any size,
+keep in mind that its [first sentence](/pkg/go/doc/#Package.Synopsis)
+will appear in godoc's [package list](/pkg/).
 
 Comments that are not adjacent to a top-level declaration are omitted from godoc's output,
 with one notable exception.

_content/doc/comment.md

@@ -49,7 +49,8 @@ For example:
 The square brackets in `[path/filepath]` create a [documentation link](#links).
 
 As can be seen in this example, Go doc comments use complete sentences.
-For a package comment, that means the first sentence begins with “Package <name>”.
+For a package comment, that means the [first sentence](/pkg/go/doc/#Package.Synopsis)
+begins with “Package <name>”.
 
 For multi-file packages, the package comment should only be in one source file.
 If multiple files have package comments, they are concatenated to form one