os: document that FileMode permission bits are Unix bits

[gopherbot]

by Mortdeus@gocos2d.org:

It would be nice to add 

`rwx(111[7]) rw(110[6]) rx(101[5]) r(100[4]) wx(011[3]) w(010[2]) x(001[1])`

as a godoc comment at

https://code.google.com/p/go/source/browse/src/pkg/os/types.go#56

[bradfitz]

Comment 1:

Why?  It's pretty cryptic and ugly.

[mortdeus]

Comment 2:

well, some gophers were confused in #go-nuts last night. 
It would just be nice to have a more descriptive comment, instead of one that assumes
all gophers understand Unix conventions.

[bradfitz]

Comment 3:

And saying `rwx(111[7]) rw(110[6]) rx(101[5]) r(100[4]) wx(011[3]) w(010[2]) x(001[1])`
would teach them Unix?
It's possible we could add some comment, but it should be mostly English.

[davecheney]

Comment 4:

@mortdeus if you would like to tackle this one that would be great.

Labels changed: added release-none, repo-main, suggested.

Status changed to HelpWanted.

[glacjay]

Comment 5:

maybe like this?
//                 read write exec
// 0777 (oct) = 0b   1    1    1    // user
//                   1    1    1    // group
//                   1    1    1    // other

[k4rtik]

Comment 6:

Since `pkg` is gone, the correct URL referenced in OP for this issue is
https://code.google.com/p/go/source/browse/src/os/types.go#56
Also, I doubt this is really an issue. Even if someone doesn't know UNIX conventions for
permissions, how much time does it take to Google them up?

[rsc]

Let's change "// permission bits" to "// Unix permission bits" and be done.

[bradfitz]

https://go-review.googlesource.com/8919