Better document profile functionality

[WardBrian]

Summary:

The use and syntax of profile blocks is under-documented. A section in the users guide would be great, especially addressing:

• profiling overhead, if any
• Are profiling blocks like regular blocks (e.g. local variables drop at the end)
• How and when to use them

This will also be helpful for #396

[rok-cesnovar]

Thanks Brian, will address this during the feature freeze.