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: Audacity - Browse Files at SourceForge.net (audacity-manual-2.0.5.zip)
If you want to install it locally (on your computer), see: Audacity Manual
(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 )