Move prose on future leap seconds to its own section

There are many methods that have this caveat, not just .from-posix

Author: zoffixznet

doc/Type/Instant.pod6

@@ -18,41 +18,45 @@ C<Instant> to a C<Duration> returns another Instant. Subtracting two C<Instant>s
 will yield a C<Duration>. Adding two C<Instant>s is explicitly disallowed. All
 other operations with Instants are undefined.
 
-=head1 Methods
-
-=head2 method from-posix
+=head1 Future Leap Seconds
 
-    method from-posix($posix, Bool $prefer-leap-second = False)
-
-Converts the POSIX timestamp C<$posix> to an Instant.
-If C<$prefer-leap-second> is C<True>, the return value will be
-the first of the two possible seconds in the case of a leap second.
-
-    say DateTime.new(Instant.from-posix(915148800, True));  # 1998-12-31T23:59:60Z
-    say DateTime.new(Instant.from-posix(915148800));        # 1999-01-01T00:00:00Z
-
-B<Note:> the method does not attempt to make any guesses about possible positions
-of leap seconds in the future and only uses the already-known leap seconds.
-This means you can get different results, depending on the compiler version you're
-using. For example, the December 31, 2016 leap second was announced in July and
-shipped with Rakudo 2016.07, so 2016.06 and earlier releases won't know about it:
+Perl 6's methods that involve knowledge of leap seconds do not
+make any guesses for leap seconds in the future and only use the already-known
+leap seconds. This means you can get different results, depending on the
+compiler version you're using. For example, the December 31, 2016 leap second
+was announced in July and shipped with Rakudo 2016.07, so 2016.06 and earlier
+releases won't know about it:
 
     $ perl6-2016.06 -e 'say Instant.from-posix: 1485726595'
     Instant:1485726631
 
     $ perl6-2016.07 -e 'say Instant.from-posix: 1485726595'
     Instant:1485726632
 
-Since a Rakudo compiler always returns 0 for future leap seconds it doesn't know
-about, you can patch your old code when new leap seconds are announced, so it
-will give correct results, regardless of what version of the compiler it's run on:
+Since a Rakudo compiler always returns 0 for future leap seconds it doesn't
+know about, you can patch your old code when new leap seconds are announced,
+so it will give correct results, regardless of what version of the compiler
+it runs on:
 
     $ perl6-2016.06 -e 'say ($*VM.version before 2016.07 ?? 1 !! 0) + Instant.from-posix: 1485726595'
     Instant:1485726632
 
     $ perl6-2016.07 -e 'say ($*VM.version before 2016.07 ?? 1 !! 0) + Instant.from-posix: 1485726595'
     Instant:1485726632
 
+=head1 Methods
+
+=head2 method from-posix
+
+    method from-posix($posix, Bool $prefer-leap-second = False)
+
+Converts the POSIX timestamp C<$posix> to an Instant.
+If C<$prefer-leap-second> is C<True>, the return value will be
+the first of the two possible seconds in the case of a leap second.
+
+    say DateTime.new(Instant.from-posix(915148800, True));  # 1998-12-31T23:59:60Z
+    say DateTime.new(Instant.from-posix(915148800));        # 1999-01-01T00:00:00Z
+
 =head2 method to-posix
 
     method to-posix()