I found myself trying to put links to other parts of the project on my index cards. There is something to be said for it, I think: when describing what a document should be about, one sometimes also has to remember what it is not about or what it leads up to or follows up on (content that is developed elsewhere in the project). Linking to that other document or its index card would be the quickest way to refer to it, and of course the link would be clickable too, for even greater convenience.
Scrivener already has a general purpose inter-document linking system—one that does not require you to be looking at a corkboard to use it: the Bookmarks inspector tab. Not only is this list of links you can establish here easily accessible no matter how you are working with the document—as you will note the large preview area below the list is an editor! That means the documents you link to will be directly accessible by virtue of the relationship you establish between them. And these will form back-links as well. The item you link to will get a Bookmark added to it for the item that linked from, making the relationship circular.
See also:
- §10.3, Project and Document Bookmarks, pg. 231.
- §13.4, Bookmarks Tab, in the chapter on the Inspector, pg. 323.
Thank you for pointing that out, but I didn’t explain myself very well. I wanted to be looking at the corkboard, and to be able to follow links from one index card to another. Example: Index card 23 would say ‘Describe A focusing on aspects B and C, but don’t give away what will be said about C in [[index card 26]].’
Then it is me that did not explain myself very well. 
I did understand that, but perhaps what is neglected in my description is that the Inspector is an omnipresent sidebar, similar to the binder. It will inspect whatever you are clicking on in the main editor, which means index cards. Hence, you can click on an index card in the corkboard and get a link browser/editor to the right.
Myself, I prefer the keyboard for most things, and for me interacting with bookmarks is as simple as typing and shifting between fields in a form. I press ⌃⌥⌘N twice to show and then activate the Bookmark list. I can type in the first letter or two of the document I wish to view, and hit Return to load it in the main editor (this is a setting I prefer, in the Behaviors: Document Links preference pane), Spacebar to load it in a Quick Reference panel, or simply Tab to jump down to the preview sidebar and edit it there.
And just so you know, the Synopsis field is plain-text. The reason for coming up with other mechanisms to do the same underlying thing you’re trying to do is that it is very unlikely that field will become a small rich text field in the future.
Thanks a lot for your renewed effort to explain this to me. Scrivener support is phenomenal.