Fixed help and added docu for namespaced branches

This commit changes the "Branches to build" help.
1.) The help has not been displayed at all due to wrong help filename. Now it is...
2.) " blank for 'any' " is not true because in fact <blank> results in "*/master" which means any branch ending with "/master"
3.) For the fix in git-client-plugin pull request 120 I added documentation.
https://github.com/jenkinsci/git-client-plugin/pull/120

Even without the git-client-plugin fix people have to be aware that the git-client-plugin (currently) only uses the last segmentent of the branch specifier. E.g. for origin/feature1/master only "master" is used which could result in a random head if you have multiple */master branches.
See https://issues.jenkins-ci.org/browse/JENKINS-20767?focusedCommentId=195996&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-195996