Add template tag and documentation
typos/edits to docs text.
Edits per code review
Add explicit arguments in docs
Just return, no need for an intermediate local...
Use InclusionTag to simplify… everything

Thanks for the push, @yakky
Move template into /cms/toolbar/, update path
Merge pull request #3595 from mkoistinen/Add_RenderPluginBlock

Add new template tag render_plugin_block