omit the .ttf extension in default font name

Per the Ploticus docs[1], font name shouldn't include the .ttf extension.
This seems to work for the PNG renderer, but causes issues with SVG files
(including the svg2png renderer).

Also clean up and extend docs for $wgTimelineSettings->fontFile.

[1] http://ploticus.sourceforge.net/doc/fonts.html

Change-Id: I03a7f99b98d78d8733ca5b93ab365a0b851d5485