Update HoverTool's documentation with field formatting, etc.

[mattpap]

This should include:

• formatting options (just copy or link documentation from NumeralTickFormatter)
• ability to reference complex field names using curly brackets, e.g. @{My Field}
• support for HTML templating

Perhaps formatting options should be moved to common location and just referenced from all other locations (there are at least three: tick formatter, hover tool and data table).