Improvement to NearsidePerspective projection documentation

[gholsclaw]

Description

The current (v0.24.1) definition of the Cartopy NearsidePerspective projection is: "Perspective view looking directly down from above a point on the globe. In this projection, the projected coordinates are x and y measured from the origin of a plane tangent to the Earth directly below the perspective point (e.g. a satellite)."

Please consider a more explicit definition that specifies the orientation of the plane, and includes the z-axis. For example: "Using Earth-based conventions, at the tangent location the +y axis of the projection plane points due north while the +x axis points due east. The +z axis completes the right-handed coordinate frame, and points from the tangent location toward the observer. Positions within this projection carry units of meters."

Please also consider generalizing the observed body to any solar system object, rather than always referencing the Earth. This could be done simply by adding the following text after "Earth": "(or other solar system body)".

[dopplershift]

Would be happy to merge a PR that improves the documentation as you suggest above.