Better meta for 'text' keys:

- Annotation 'text' includes a description of HTML tags and a few examples
- 'text' in now included in Histogram
- better description for 'text' (Scatter/Bar/Histogram), 'textpostion' and 'textfont'
- example for 'scl' is now shorter, to fit on webpage.
- available font family match revised list (refer to  GUI)
I chose to only include a discussion of HTML escape characters in Annotations. 

Moreover, I did not see the point of writing out any `\n ` is converted to `<br>` in the graph reference. I think @theengineear's  [warning](https://github.com/plotly/python-api/commit/62c38d6bfbba895fdd28ccc7303f7252be214919) should be good enough.