encoding/json: clarify omitempty option for {array,slice,map,string}

This CL is inspired by:
#29310 (comment)

When I read omitempty option in encoding/xml package, I find it's
a bit different than encoding/json package.

I think it's more precise to say:
"any array, slice, map, or string of length zero."

Update #29310

Change-Id: Ia77167c3155411640224b349d4b34d0bb91ee11e
GitHub-Last-Rev: a4cf00d
GitHub-Pull-Request: #69984
Reviewed-on: https://go-review.googlesource.com/c/go/+/621835
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/encoding/json/encode.go

@@ -72,8 +72,8 @@ import (
 //
 // The "omitempty" option specifies that the field should be omitted
 // from the encoding if the field has an empty value, defined as
-// false, 0, a nil pointer, a nil interface value, and any empty array,
-// slice, map, or string.
+// false, 0, a nil pointer, a nil interface value, and any array,
+// slice, map, or string of length zero.
 //
 // As a special case, if the field tag is "-", the field is always omitted.
 // Note that a field with name "-" can still be generated using the tag "-,".