Clarify polygon orientation requirement for buffer algorithm

[kashish2710]

This PR adds a note in the buffer documentation (under [heading Strategies]) to clarify that the buffer algorithm expects polygons to be counter-clockwise. If a polygon is clockwise, it should be reversed using:

boost::geometry::reverse(poly);

A link to the Polygon Concept is also provided for more details.

This helps users avoid confusion when using buffer with custom polygons. Related issue: #1375