golang切片，用slice切片[a：b：c]

I read go slice usage and internals and Slice and Effective go#slice but there is nothing about slicing a slice with 3 number like this : slice[a:b:c]

For example this code :

package main

import "fmt"

func main() {
    var s = []string{"a", "b", "c", "d", "e", "f", "g"}
    fmt.Println(s[1:2:6], len(s[1:2:6]), cap(s[1:2:6]))
    fmt.Println(s[1:2:5], len(s[1:2:5]), cap(s[1:2:5]))
    fmt.Println(s[1:2], len(s[1:2]), cap(s[1:2]))

}

go playground result is this :

[b] 1 5
[b] 1 4
[b] 1 6

I can understand that the third one is something about capacity, but what is the exact meaning of this?
Do I miss something in documents?

The syntax has been introduced in Go 1.2, as I mentioned in "Re-slicing slices in Golang".
It is documented in Full slice expressions:

a[low : high : max]

constructs a slice of the same type, and with the same length and elements as the simple slice expression a[low : high].
Additionally, it controls the resulting slice's capacity by setting it to max - low.
Only the first index may be omitted; it defaults to 0.

After slicing the array a:

a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]

the slice t has type []int, length 2, capacity 4, and elements

t[0] == 2
t[1] == 3

The design document for that feature had the following justification:

It would occasionally be useful, for example in custom []byte allocation managers, to be able to hand a slice to a caller and know that the caller cannot edit values beyond a given subrange of the true array.

The addition of append to the language made this somewhat more important, because append lets programmers overwrite entries between len and cap without realizing it or even mentioning cap.