Update docs and tooltips for two-clip Join Extend

Add source_clip_2 and original_clip_2 to README input tables, update
Join Extend mode reference with two-clip diagram, update MergeBack
description and behavior section, fix mode output type label.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

merge_node.py

@@ -90,6 +90,7 @@ Pass-through modes (Edge Extend, Frame Interpolation, Keyframe, Video Inpaint):
 Splice modes (End, Pre, Middle, Join, Bidirectional, Replace):
   Reconstructs original[:trim_start] + vace_output + original[trim_end:]
   with automatic blending across the full context zones.
+  For two-clip Join Extend, connect original_clip_2 — the tail comes from the second clip.
 
 Blend methods:
   none          — Hard cut at seams (fastest)