(file-precious-flag): Doc clarification.

lisp/files.el

@@ -131,7 +131,13 @@ If the buffer is visiting a new file, the value is nil.")
 (defconst file-precious-flag nil
   "*Non-nil means protect against I/O errors while saving files.
 Some modes set this non-nil in particular buffers.
-Note that this forces backups to be made by copying.
+
+This feature works by writing the new contents into a temporary file
+and then renaming the temporary file to replace the original.
+In this way, any I/O error in writing leaves the original untouched,
+and there is never any instant where the file is nonexistent.
+
+Note that this feature forces backups to be made by copying.
 Yet, at the same time, saving a precious file
 breaks any hard links between it and other files.")