golang slice, slicing a slice with slice[a:b:c]

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.

It is explained well in Go release notes. Please note a missing first index ([:0:0]) defaults to zero.