After a lot of fiddling about and testing the possibilities I’ve determined that the only plugin location that works for me is…
~/.audacity-files/plug-ins
…but the manual (How do I install plugins on Linux) says to use…
~/.audacity-files/Plug-Ins
~/.audacity-data/Plug-Ins
Ubuntu 14.04.2
Audacity 2.0.5
It’s frustrating that such basic info can be wrong. Can it be corrected? Or is it a version thing?
(/usr/share/audacity/plug-ins also works of course, but I prefer a user install)
It’s a version thing. You are looking at the Audacity 2.1.0 manual.
Audacity 2.1.0 supports more locations for plugins. I’m currently using ~/.audacity-files/Plug-Ins for Nyquist plugins and ~/.audacity-data/Plug-Ins for LV2 plugins (which I find convenient as I have a lot of Nyquist plugins).
The 2.0.5 manual may be downloaded as a ZIP file from here: https://sourceforge.net/projects/audacity/files/audacity/2.0.5/ (audacity-manual-2.0.5.zip)
If you want to install it locally (on your computer), see: http://manual.audacityteam.org/o/man/unzipping_the_manual.html
(if you do install locally, double check that you install it in the correct location and then it will open from the link in the Audacity Help menu - it’s easy to get the location wrong - don’t ask me how I know that 
)