reflect: Slice documentation should note that an array must be addressable

[kortschak]

What steps will reproduce the problem?
If possible, include a link to a program on play.golang.org.
1. Navigate to http://tip.golang.org/pkg/reflect/#Value.Slice

What is the expected output?
Inclusion of a notice that Slice cannot be called on a unaddressable array
reflect.Value. 

What do you see instead?
No such warning.

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
Linux

Which version are you using?  (run 'go version')
go1.0.3/tip

Please provide any additional information below.

[kortschak]

Comment 1:

Proposed CL: https://golang.org/cl/7239045

[rsc]

Comment 2:

This issue was closed by revision 11d16dc.

Status changed to Fixed.