patch 9.0.1990: strange error number

Problem:  strange error number
Solution: change error number,
          add doc tag for E1507

closes: #13270

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Christ van Willegen <cvwillegen@gmail.com>

runtime/doc/builtin.txt

@@ -6884,6 +6884,14 @@ printf({fmt}, {expr1} ...)				*printf()*
 <			E1505: Invalid format specifier:
 			%1$d at width %2$d is: %01$*2$.3$d
 
+							*E1507*
+		This internal error indicates that the logic to
+		parse a positional format error ran into a problem
+		that couldn't be otherwise reported. Please file a
+		bug against vim if you run into this, copying the
+		exact format string and parameters that were used.
+
+
 prompt_getprompt({buf})					*prompt_getprompt()*
 		Returns the effective prompt text for buffer {buf}.  {buf} can
 		be a buffer name or number.  See |prompt-buffer|.