doc/go1.14: document io/ioutil.TempDir's predictable prefix+suffix
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,akutz/go,cloudflare/go,golang/go,nawawi/go,golang/go,akutz/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go,akutz/go