docs(extmarks): add undocumented "details" array fields

src/nvim/api/extmark.c

@@ -215,8 +215,8 @@ ArrayOf(Integer) nvim_buf_get_extmark_by_id(Buffer buffer, Integer ns_id,
   return extmark_to_array(extmark, false, details, hl_name);
 }
 
-/// Gets |extmarks| (including |signs|) in "traversal order" from a |charwise|
-/// region defined by buffer positions (inclusive, 0-indexed |api-indexing|).
+/// Gets |extmarks| in "traversal order" from a |charwise| region defined by
+/// buffer positions (inclusive, 0-indexed |api-indexing|).
 ///
 /// Region can be given as (row,col) tuples, or valid extmark ids (whose
 /// positions define the bounds). 0 and -1 are understood as (0,0) and (-1,-1)
@@ -234,6 +234,10 @@ ArrayOf(Integer) nvim_buf_get_extmark_by_id(Buffer buffer, Integer ns_id,
 /// the `overlap` option might be useful. Otherwise only the start position
 /// of an extmark will be considered.
 ///
+/// Note: legacy signs placed through the |:sign| commands are implemented
+/// as extmarks and will show up here. Their details array will contain a
+/// `sign_name` field.
+///
 /// Example:
 ///
 /// ```lua
@@ -434,7 +438,9 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id, Object start, Object e
 ///                   if text around the mark was deleted and then restored by undo.
 ///                   Defaults to true.
 ///               - invalidate : boolean that indicates whether to hide the
-///                   extmark if the entirety of its range is deleted. If
+///                   extmark if the entirety of its range is deleted. For
+///                   hidden marks, an "invalid" key is added to the "details"
+///                   array of |nvim_buf_get_extmarks()| and family. If
 ///                   "undo_restore" is false, the extmark is deleted instead.
 ///               - priority: a priority value for the highlight group or sign
 ///                   attribute. For example treesitter highlighting uses a