+You may use references to environment variables in the values of parameters.
+String parameters will have %VAR% references expanded as late as possible (when
+the string is used in the program), allowing for the use of variables that
+rsync sets at connection time, such as RSYNC_USER_NAME. Non-string parameters
+(such as true/false settings) are expanded when read from the config file. If
+a variable does not exist in the environment, or if a sequence of characters is
+not a valid reference (such as an un-paired percent sign), the raw characters
+are passed through unchanged. This helps with backward compatibility and
+safety (e.g. expanding a non-existent %VAR% to an empty string in a path could
+result in a very unsafe path). Anyone that needs to insert a literal % string
+into a value should use %%.
+