Fix replay recording temporary paths not supporting subdirectories (#19887)

Content.Shared/CCVar/CCVars.cs

@@ -1776,6 +1776,7 @@ namespace Content.Shared.CCVar
         /// <summary>
         /// Path that, if provided, automatic replays are initially recorded in.
         /// When the recording is done, the file is moved into its final destination.
+        /// Unless this path is rooted, it will be relative to <see cref="CVars.ReplayDirectory"/>.
         /// </summary>
         public static readonly CVarDef<string> ReplayAutoRecordTempDir =
             CVarDef.Create("replay.auto_record_temp_dir", "", CVar.SERVERONLY);