document how to load package with only extra Rd tags

[maxheld83]

I stubbed my toe on this and thought others might as well. For example, #1413 raises similar issues.

I was confused specifically what to do if my roxygen-extending packageFoo only adds new .Rd tags (roxy_tag_*() methods), but didn't have nor need a new roclet.

The solution is hidden pretty well the load_options() documentation and this release note.