futex.2: Clarify which operations were available as at Linux 2.6.0

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/futex.2

@@ -147,7 +147,8 @@ The operation specified in
 .I op
 is one of the following:
 .TP
-.B FUTEX_WAIT
+.BR FUTEX_WAIT " (since Linux 2.6.0)"
+.\" Strictly speaking, since some time in 2.5.x
 This operation atomically verifies that the futex address
 .I uaddr
 still contains the value
@@ -184,7 +185,8 @@ operation.
 .\" FIXME TO complete
 [As yet undocumented]
 .TP
-.B FUTEX_WAKE
+.BR FUTEX_WAKE " (since Linux 2.6.0)"
+.\" Strictly speaking, since Linux 2.5.x
 This operation wakes at most \fIval\fP
 processes waiting on this futex address (i.e., inside
 .BR FUTEX_WAIT ).
@@ -227,7 +229,8 @@ to 1 (indicating that it is available).
 .\" FIXME to complete
 [As yet undocumented]
 .TP
-.BR FUTEX_FD " (present up to and including Linux 2.6.25)"
+.BR FUTEX_FD " (from Linux 2.6.0 up to and including Linux 2.6.25)"
+.\" Strictly speaking, from Linux 2.5.x to 2.6.25
 To support asynchronous wakeups, this operation associates a file descriptor
 with a futex.
 .\" , suitable for .BR poll (2).
@@ -253,7 +256,8 @@ Because it was inherently racy,
 .B FUTEX_FD
 has been removed from Linux 2.6.26 onward.
 .TP
-.BR FUTEX_REQUEUE " (since Linux 2.5.70)"
+.BR FUTEX_REQUEUE " (since Linux 2.6.0)"
+.\" Strictly speaking: from Linux 2.5.70
 This operation was introduced in order to avoid a "thundering herd" effect
 when
 .B FUTEX_WAKE
@@ -453,6 +457,9 @@ was not equal to the expected value
 at the time of the call.
 .SH VERSIONS
 .PP
+Futexes were first made available in a stable kernel release
+with Linux 2.6.0.
+
 Initial futex support was merged in Linux 2.5.7 but with different semantics
 from what was described above.
 A 4-argument system call with the semantics