doc/go1.14: document io/ioutil.TempDir's predictable prefix+suffix
    
    Documents io/ioutil.TempDir's new ability to create
    predictable prefixes and suffixes, derived from the argument
    'pattern', separated by the last '*' in it.
    
    References: CL 198488
    
    Updates #36878
    
    Change-Id: I92c52fcc7d480ce74746e99e6e85a04efb87294f
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217780
    Reviewed-by: Ian Lance Taylor <iant@golang.org>