change sync content transition plan and fine tune warning

Only display warning when git-annex sync (without --content or
--no-content) is used with repositories that have preferred content
configured.

Sponsored-by: Leon Schuermann on Patreon

doc/bugs/Changing_sync_to_include_content_breaks_UX.mdwn

@@ -64,3 +64,5 @@ Absolutely.  I've been using git-annex since nearly the beginning; and use it ex
 Thanks for writing git-annex.  It's the reason I've been sponsoring you on Patreon for years.
 
 Please don't break backwards compatibility.  Even in the user experience :-)
+
+> [[closing|done]] per new plan --[[Joey]]

doc/bugs/Changing_sync_to_include_content_breaks_UX/comment_19_bdc211b026d3b05de127f6ed5a3d19b0._comment

@@ -0,0 +1,21 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 19"""
+ date="2023-08-14T16:56:48Z"
+ content="""
+Changing the default preferred content expression to `present` rather than
+`anything` would affect other things like `get --auto` and the assistant. I
+don't think that is a good idea.
+
+My idea in comment #15 is a special case for sync and not a wider change.
+
+It will have a special case either way. The current special case means
+that users who have set preferred content need to remember to use `sync
+--content`. The new special case will only manifest as a difference in
+behavior between `sync` and `sync --content` (and `get --auto` etc)
+for users with no preferred content configured.
+
+I do think that's a valuable narrowing of the special case.
+
+Gone ahead and changed the warning.
+"""]]