docs: Improve documentation of ensure final new line on save (#25960)

The function ensure_final_newline in buffer.rs has this explanation:
Ensures that the buffer ends with a single newline character, no other
whitespace.

The documentation wasn't explaining well that we actually remove any
lines containing only whitespace and keep only 1 line at the end of a
buffer.

Release Notes:

- N/A

---------

Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Danilo Leal <danilo@zed.dev>

assets/settings/default.json

@@ -720,8 +720,8 @@
   "remove_trailing_whitespace_on_save": true,
   // Whether to start a new line with a comment when a previous line is a comment as well.
   "extend_comment_on_newline": true,
-  // Whether or not to ensure there's a single newline at the end of a buffer
-  // when saving it.
+  // Removes any lines containing only whitespace at the end of the file and
+  // ensures just one newline at the end.
   "ensure_final_newline_on_save": true,
   // Whether or not to perform a buffer format before saving
   //

docs/src/configuring-zed.md

@@ -917,7 +917,7 @@ Each option controls displaying of a particular toolbar element. If all elements
 
 ## Ensure Final Newline On Save
 
-- Description: Whether or not to ensure there's a single newline at the end of a buffer when saving it.
+- Description: Removes any lines containing only whitespace at the end of the file and ensures just one newline at the end.
 - Setting: `ensure_final_newline_on_save`
 - Default: `true`