Merge branch 'uk/merge-subtree-doc-update'

Belated documentation update to adjust to a new world order that
happened a yew years ago.

* uk/merge-subtree-doc-update:
  howto/using-merge-subtree: mention --allow-unrelated-histories

Documentation/howto/using-merge-subtree.txt

@@ -33,7 +33,7 @@ Here is the command sequence you need:
 
 ----------------
 $ git remote add -f Bproject /path/to/B <1>
-$ git merge -s ours --no-commit Bproject/master <2>
+$ git merge -s ours --no-commit --allow-unrelated-histories Bproject/master <2>
 $ git read-tree --prefix=dir-B/ -u Bproject/master <3>
 $ git commit -m "Merge B project as our subdirectory" <4>