os: clarify meaning of "truncate"

[UlisseMini]

For non-native speakers the term "empty" might be easier to understand
eg change // truncate regular writable file when opened. to // truncate (empty) regular writable file when opened. in the O_TRUNC constant

[gopherbot]

Change https://golang.org/cl/156121 mentions this issue: os: clarify meaning of "truncate"

[bradfitz]

Sorry, this adds more confusion than it helps: "empty" can be both a verb or an adjective.

Let's leave it as-is. People can look up the word. It's the same word used elsewhere in Unix/POSIX/etc.

[UlisseMini]

Alright i understand :)