Merge pull request rpm-software-management#56 from dmnks/bz1411692

docs: clarify variable name matching

Author: megaumi

docs/yum.conf.5

@@ -1340,6 +1340,13 @@ the same name. If the shell environment variable does not exist then the
 configuration file variable will not be replaced.
 
 .LP
+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