diff -up yum-3.4.3/docs/yum.conf.5.orig yum-3.4.3/docs/yum.conf.5
--- yum-3.4.3/docs/yum.conf.5.orig	2017-11-01 14:58:28.259740017 +0100
+++ yum-3.4.3/docs/yum.conf.5	2017-11-01 14:58:48.528648100 +0100
@@ -1356,8 +1356,17 @@ the same name. If the shell environment
 configuration file variable will not be replaced.
 
 .LP
-As of 3.2.28, any file in /etc/yum/vars is turned into a variable named after
-the filename (or overrides any of the above variables).
+When variable names are parsed in a string, all alphanumeric characters and
+underscores immediately following a $ sign are interpreted as part of a name.
+If a variable is undefined, it will not be replaced.
+For example, the strings $releasever-foo or $releasever/foo will be expanded
+with the $releasever value accordingly, whereas $releaseverfoo or
+$releasever_foo will not be expanded.
+
+As of 3.2.28, any properly named file in /etc/yum/vars is turned into
+a variable named after the filename (or overrides any of the above variables).
+Filenames may contain only alphanumeric characters and underscores
+and be in lowercase.
 
 Note that no warnings/errors are given if the files are unreadable, so creating
 files that only root can read may be confusing for users.