document link

[Fil]

for #1343

note: more precise jsdoc for the curve option

todo:

• maybeSameValue

[mbostock]

Add documentation for the maybeSameValue defaults?

[Fil]

Unnecessary repetition of instead:

   * If the plot uses a spherical **projection**, the default *auto* **curve**
   * will render links as geodesics; to draw a straight line instead, use the
   * *linear* **curve** instead.

[mbostock]

Thanks, I will fix.