[DOCBUG] git subtree synopsis needs updating

As the examples in git-subtree.txt show, the synopsis in the same file should surely get a patch along the lines of:

-'git subtree' add -P <prefix> <commit> +'git subtree' add -P <prefix> <repository> <commit>

Failure to specify the repository (by just specifying a local commit) fails with the cryptic:

 warning: read-tree: emptying the index with no arguments is deprecated; use --empty
 fatal: just how do you expect me to merge 0 trees?

Furthermore, the doc paragraph for add, aside from mentionning <repository>, also mentions a <refspec> which the synopsis does not show either.

As a sidenote it someone wants to do some maintainance, using "." as repository when the branch to subtree-add is already locally available does not work well either (fails with "could not find ref myremote/myhead").