Hello.

This is what the patch does:

1. Document existing command `ffap-other-tab'.
2. Create `find-file-read-only-other-tab' and `ffap-read-only-other-tab'.
3. Bind "\C-r" to `find-file-read-only-other-tab', like how "\C-f" is
    bound to `find-file-other-tab'.
4. Document `ffap-read-only-other-tab' in misc.texi.

I don't really know the Texinfo format, so I just copied the mark-up used for 
other items.

Is there anything that should be changed?

Thank you.