Clarify margin documentation

[gravitystorm]

The documentation for shield-margin is:

Minimum distance that a shield can be placed from any other text, shield, or marker.

The documentation for text-margin is:

Minimum distance that a label can be placed from any other text, shield, or marker.

Neither is clear on how margins interact with objects drawn by the PointSymbolizer. I guess that it should read "text, shield, point or marker" but it would be good to both a) confirm this is true and b) update the documentation to clarify.