Is there a detailed user's documentation for KDE Plasma 5?

The place for all KDE related User documentation is found at KDE Userbase. If you feel there are changes that should be made based on your experiences, there are a few things you can do, listed in descending order of relevance:

(1) Edit the Wiki yourself, adding the specific information you've found. Information can be found here. Also see here.

(2) Contact the documentation group via IRC: irc://irc.kde.org/kde-docs

(3) Post a message on the Contributor's Corner section of the KDE Forums asking for the changes to be made or how you can most easily make them.

Help with documentation is ALWAYS needed and appreciated.