#726 Document line style of tics (resolves bug 1730)

[Harald Harders]

Update of the gnuplot documentation (gnuplot.doc) in order to explain that the border line style is used for tics even if they are drawn at the axes (fixed bug 1730, Line style of tics at the axis).

As a side effect, this patch fixes and outdated documentation of line styles (dash type and line color missing) in some other places.

[Ethan Merritt]

Applied.
Thanks.