WIP: document link resolution behavior and design decisions

This commit is starting as attempt to add useful context for link functions
in the comments / docstrings, and clean up a few things for better
readability.  This is preliminary work for a solution to:

https://github.com/vimwiki/vimwiki/issues/1271

...which is gradually becoming a description of everything that's wrong
with how VimWiki links to files, more or less.

It includes a decision document, which should become a full description
of the problem and steps taken.

This commit _will_ be rewritten.

Bug: #1271

doc/design_notes.md

@@ -4,6 +4,9 @@ This file is meant to document design decisions and algorithms inside Vimwiki
 which are too large for code comments, and not necessarily interesting to
 users. Please create a new section to document each behavior.
 
+## Link resolution
+
+See [VimWiki file link resolution](./2023-03-23-file-link-resolution.wiki).
 
 ## Formatting tables