"Save current buffer in visited file if modified. Variations are described below.
By default, makes the previous version into a backup file
if previously requested or if this is the first save.
-Combined with one \\[universal-argument], marks this version
+Prefixed with one \\[universal-argument], marks this version
to become a backup when the next save is done.
-Combined with two \\[universal-argument]'s,
+Prefixed with two \\[universal-argument]'s,
unconditionally makes the previous version into a backup file.
-Combined with three \\[universal-argument]'s, marks this version
+Prefixed with three \\[universal-argument]'s, marks this version
to become a backup when the next save is done,
and unconditionally makes the previous version into a backup file.
-With argument of 0, never make the previous version into a backup file.
+With a numeric argument of 0, never make the previous version
+into a backup file.
If a file's name is FOO, the names of its numbered backup versions are
FOO.~i~ for various integers i. A non-numbered backup file is called FOO~.