feat(mini.nvim): sync to 5121416

Author: echasnovski

mini.nvim/doc/mini-diff.qmd

@@ -283,7 +283,8 @@ used in custom statusline to show an overview of hunks in current buffer:
 
 - `vim.b.minidiff_summary` is a table with the following fields:
 
-    - `source_name` - name of the active source.
+    - `source_name` - name of the active source. This is the only present field
+      if buffer's reference text is not (yet) set.
 
     - `n_ranges` - number of hunk ranges (sequences of contiguous hunks).
 
@@ -295,10 +296,10 @@ used in custom statusline to show an overview of hunks in current buffer:
 
 
 - `vim.b.minidiff_summary_string` is a string representation of summary
-  with a fixed format. It is expected to be used as is. To achieve
-  different formatting, use `vim.b.minidiff_summary` to construct one.
-  The best way to do this is by overriding `vim.b.minidiff_summary_string`
-  in the callback for [MiniDiff-update-event](mini-diff.qmd#minidiff-update-event) event:
+  with a fixed format. Empty string if there is no reference text (yet).
+  It is expected to be used as is. To achieve different formatting, use
+  `vim.b.minidiff_summary` to construct one. The best way to do this is by
+  overriding `vim.b.minidiff_summary_string` inside [MiniDiff-update-event](mini-diff.qmd#minidiff-update-event):
 
   ```lua
   local format_summary = function(data)