patch 8.2.4720: ABB Rapid files are not recognized properly

Problem:    ABB Rapid files are not recognized properly.
Solution:   Add checks for ABB Rapid files. (Patrick Meiser-Knosowski,
            closes #10104)
diff --git a/runtime/doc/filetype.txt b/runtime/doc/filetype.txt
index 0274a73..b921c3d 100644
--- a/runtime/doc/filetype.txt
+++ b/runtime/doc/filetype.txt
@@ -142,6 +142,7 @@
 	*.asm		g:asmsyntax	|ft-asm-syntax|
 	*.asp		g:filetype_asp	|ft-aspvbs-syntax| |ft-aspperl-syntax|
 	*.bas		g:filetype_bas	|ft-basic-syntax|
+	*.cfg		g:filetype_cfg
 	*.dat		g:filetype_dat
 	*.frm		g:filetype_frm	|ft-form-syntax|
 	*.fs		g:filetype_fs	|ft-forth-syntax|