all: add a few more godoc links

Over the past few months as I read the standard library's documentation
I kept finding spots where godoc links would have helped me.
I kept adding to a stash of changes to fix them up bit by bit.

The stash has grown big enough by now, and we're nearing a release,
so I think it's time to merge to avoid git conflicts or bit rot.

Note that a few sentences are slightly reworded,
since "implements the Fooer interface" can just be "implements [Fooer]"
now that the link provides all the context needed to the user.

Change-Id: I01c31d3d3ff066d06aeb44f545f8dd0fb9a8d998
Reviewed-on: https://go-review.googlesource.com/c/go/+/508395
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/encoding/json/encode.go

@@ -28,14 +28,14 @@ import (
 // Marshal returns the JSON encoding of v.
 //
 // Marshal traverses the value v recursively.
-// If an encountered value implements the Marshaler interface
-// and is not a nil pointer, Marshal calls its MarshalJSON method
-// to produce JSON. If no MarshalJSON method is present but the
-// value implements encoding.TextMarshaler instead, Marshal calls
-// its MarshalText method and encodes the result as a JSON string.
+// If an encountered value implements [Marshaler]
+// and is not a nil pointer, Marshal calls [Marshaler.MarshalJSON]
+// to produce JSON. If no [Marshaler.MarshalJSON] method is present but the
+// value implements [encoding.TextMarshaler] instead, Marshal calls
+// [encoding.TextMarshaler.MarshalText] and encodes the result as a JSON string.
 // The nil pointer exception is not strictly necessary
 // but mimics a similar, necessary exception in the behavior of
-// UnmarshalJSON.
+// [Unmarshaler.UnmarshalJSON].
 //
 // Otherwise, Marshal uses the following type-dependent default encodings:
 //