Improvements to API documentation.

* Added requirement to send header when POSTing config.xml.
* Don't tell anyone with Overall/Read where JENKINS_HOME is located
* Add an alternative to viewing config.xml files on the server for non-admins

[FIXED JENKINS-19898] test base classes should not be linked to the junit implementation.

fixed misspelling in the Polish translation

[FIXED JENKINS-17606] Reuse existing fingerprint action if present.

Prevents multiple FingerprintActions.

[FIXED JENKINS-20730] Use correct folder in log message

[FIXED JENKINS-21023] Rewrote WorkspaceCleanupThread to determine where workspaces for known items should be, rather than looking for directories that look like they might be workspaces.
One key advantage is that this correctly handles the new default workspace location on the master.
Another is that this allows folders to be correctly skipped, and jobs in folders to be correctly deleted.
A subtler advantage would be handling of configured nondefault workspace locations, and compatibility with WorkspaceLocator.
(Orphaned workspaces from deleted jobs are not removed, but these were not removed before either.)

Since sorting of plugins in the update manager is based on displayName
this name should not have a common prefix otherwise it's hard to find
a given plugin name.

Update Messages_ru.properties

Fixed typo. Unstable in russian is "Нестабильно", not "Настабильно".