patch 9.0.2064: cannot use buffer-number for errorformat

Problem:  cannot use buffer-number for errorformat
Solution: add support for parsing a buffer number using '%b' in
          'errorformat'

closes: #13419

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/runtime/doc/quickfix.txt b/runtime/doc/quickfix.txt
index 96ba382..cbfd791 100644
--- a/runtime/doc/quickfix.txt
+++ b/runtime/doc/quickfix.txt
@@ -1422,6 +1422,7 @@
 Basic items
 
 	%f		file name (finds a string)
+	%b		buffer number (finds a number)
 	%o		module name (finds a string)
 	%l		line number (finds a number)
 	%e		end line number (finds a number)
@@ -1461,6 +1462,11 @@
 This means that a file name which is a single alphabetical letter will not be
 detected.
 
+The "%b" conversion is used to parse a buffer number.  This is useful for
+referring to lines in a scratch buffer or a buffer with no name.  If a buffer
+with the matching number doesn't exist, then that line is used as a non-error
+line.
+
 The "%p" conversion is normally followed by a "^".  It's used for compilers
 that output a line like: >
 	    ^