runtime: add more verbose comments in Caller method

[minaevmike]

https://github.com/golang/go/blob/master/src/runtime/extern.go#L179
1+skip-1 makes no sense, i think i can be converted to skip or some comments can be added to discribe why we make +1-1.

[ianlancetaylor]

For the record, the -1 was added in https://golang.org/cl/4810054, when the code was changed to ask for two PC values. At this point we should probably just drop it.

[wingyplus]

So we can remove +1-1 in L179. Am I right?

[ianlancetaylor]

Yes.

[gopherbot]

Change https://golang.org/cl/124895 mentions this issue: runtime: remove +1-1 when asking PC values