- "Write current buffer into file FILENAME using some FORMAT.
-Make buffer visit that file and set the format as the default for future
-saves. If the buffer is already visiting a file, you can specify a directory
-name as FILENAME, to write a file of the same old name in that directory.
-
-If optional third arg CONFIRM is non-nil, ask for confirmation before
-overwriting an existing file. Interactively, confirmation is required
+ "Write current buffer into FILENAME, using a format based on FORMAT.
+Constructs the actual format starting from FORMAT, then appending
+any elements from the value of `buffer-file-format' with a non-nil
+`preserve' flag (see the documentation of `format-alist'), if they
+are not already present in FORMAT. It then updates `buffer-file-format'
+with this format, making it the default for future saves.
+
+If the buffer is already visiting a file, you can specify a
+directory name as FILENAME, to write a file of the same old name
+in that directory.
+
+If optional third arg CONFIRM is non-nil, asks for confirmation before
+overwriting an existing file. Interactively, requires confirmation