os: document that os.CreateTemp uses permissions 0600

[jech]

Go version

go version go1.22.1 linux/amd64

Output of go env in your module/workspace:

Irrelevant, but happy to provide if required.

What did you do?

I checked the documentation for os.CreateTemp. I couldn't find what permissions were used for the file. I had to check the source in order to find out that it's 0600 (and rightly so).

What did you see happen?

go doc os.CreateTemp | grep -i permission

What did you expect to see?

An indication that the file is not readable by other users, (and that it's guaranteed to remain that way in future Go versions).

[gopherbot]

Change https://go.dev/cl/578395 mentions this issue: os: document file mode of temporary files

[cagedmantis]

Tagging owners even though @ianlancetaylor mailed a CL.

@rsc @robpike @ianlancetaylor @bradfitz @griesemer