Clarify that get_unix_time() returns seconds

(cherry picked from commit 6e4a8b7f37e0bd934202145a6b2c0f283a14a877)

doc/classes/OS.xml

@@ -537,7 +537,7 @@
 			<return type="int">
 			</return>
 			<description>
-				Returns the current UNIX epoch timestamp.
+				Returns the current UNIX epoch timestamp in seconds.
 				[b]Important:[/b] This is the system clock that the user can manully set. [b]Never use[/b] this method for precise time calculation since its results are also subject to automatic adjustments by the operating system. [b]Always use[/b] [method get_ticks_usec] or [method get_ticks_msec] for precise time calculation instead, since they are guaranteed to be monotonic (i.e. never decrease).
 			</description>
 		</method>