unix: drop references to Unix epoch in Timeval/Timespec docs

Follow what CL 277073 did for package syscall:

The various conversion functions just change the format of time values.
They don't use the Unix epoch. Although in practice the values are often
times since the Unix epoch, they aren't always, so referring to the
epoch can be confusing.

Updates golang/go#43010

Change-Id: Icb9953e20d7cab8e36ddfb467dc2380ee181f18b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/277252
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/timestruct.go

@@ -8,12 +8,10 @@ package unix
 
 import "time"
 
-// TimespecToNsec converts a Timespec value into a number of
+// TimespecToNSec returns the time stored in ts as nanoseconds.
-// nanoseconds since the Unix epoch.
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 
-// NsecToTimespec takes a number of nanoseconds since the Unix epoch
+// NsecToTimespec converts a number of nanoseconds into a Timespec.
-// and returns the corresponding Timespec value.
 func NsecToTimespec(nsec int64) Timespec {
 	sec := nsec / 1e9
 	nsec = nsec % 1e9
@@ -42,12 +40,10 @@ func TimeToTimespec(t time.Time) (Timespec, error) {
 	return ts, nil
 }
 
-// TimevalToNsec converts a Timeval value into a number of nanoseconds
+// TimevalToNsec returns the time stored in tv as nanoseconds.
-// since the Unix epoch.
 func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
 
-// NsecToTimeval takes a number of nanoseconds since the Unix epoch
+// NsecToTimeval converts a number of nanoseconds into a Timeval.
-// and returns the corresponding Timeval value.
 func NsecToTimeval(nsec int64) Timeval {
 	nsec += 999 // round up to microsecond
 	usec := nsec % 1e9 / 1e3
@@ -59,24 +55,22 @@ func NsecToTimeval(nsec int64) Timeval {
 	return setTimeval(sec, usec)
 }
 
-// Unix returns ts as the number of seconds and nanoseconds elapsed since the
+// Unix returns the time stored in ts as seconds plus nanoseconds.
-// Unix epoch.
 func (ts *Timespec) Unix() (sec int64, nsec int64) {
 	return int64(ts.Sec), int64(ts.Nsec)
 }
 
-// Unix returns tv as the number of seconds and nanoseconds elapsed since the
+// Unix returns the time stored in tv as seconds plus nanoseconds.
-// Unix epoch.
 func (tv *Timeval) Unix() (sec int64, nsec int64) {
 	return int64(tv.Sec), int64(tv.Usec) * 1000
 }
 
-// Nano returns ts as the number of nanoseconds elapsed since the Unix epoch.
+// Nano returns the time stored in ts as nanoseconds.
 func (ts *Timespec) Nano() int64 {
 	return int64(ts.Sec)*1e9 + int64(ts.Nsec)
 }
 
-// Nano returns tv as the number of nanoseconds elapsed since the Unix epoch.
+// Nano returns the time stored in tv as nanoseconds.
 func (tv *Timeval) Nano() int64 {
 	return int64(tv.Sec)*1e9 + int64(tv.Usec)*1000
 }