Document uniqueness limitation of ‘format’

* doc/lispref/strings.texi (Formatting Strings):
* src/editfns.c (Fformat):
Document that field numbers should be unique within a format.

doc/lispref/strings.texi

@@ -965,9 +965,10 @@ extra values to be formatted are ignored.
 decimal number immediately after the initial @samp{%}, followed by a
 literal dollar sign @samp{$}.  It causes the format specification to
 convert the argument with the given number instead of the next
-argument.  Field numbers start at 1.  A format can contain either
-numbered or unnumbered format specifications but not both, except that
-@samp{%%} can be mixed with numbered specifications.
+argument.  Field numbers start at 1.  A field number should differ
+from the other field numbers in the same format.  A format can contain
+either numbered or unnumbered format specifications but not both,
+except that @samp{%%} can be mixed with numbered specifications.
 
 @example
 (format "%2$s, %3$s, %%, %1$s" "x" "y" "z")

src/editfns.c

@@ -3901,9 +3901,10 @@ where field is [0-9]+ followed by a literal dollar "$", flags is
 followed by [0-9]+.
 
 If a %-sequence is numbered with a field with positive value N, the
-Nth argument is substituted instead of the next one.  A format can
-contain either numbered or unnumbered %-sequences but not both, except
-that %% can be mixed with numbered %-sequences.
+Nth argument is substituted instead of the next one.  A field number
+should differ from the other field numbers in the same format.  A
+format can contain either numbered or unnumbered %-sequences but not
+both, except that %% can be mixed with numbered %-sequences.
 
 The + flag character inserts a + before any positive number, while a
 space inserts a space before any positive number; these flags only