Be more specific about what expand-polygon can do

As discussed in horizon-eda/horizon-pool-convention#13, it's complicated and I made some incorrect assumptions after reading this. I think making it clear here will reach anybody who goes looking.

The question of how much detail to specify in horizon-pool-convention seems separate to me, because the docs for the parameter program commands should be clear on their own.

parameter-programs.rst

@@ -146,7 +146,10 @@ expand-polygon
 
 ``expand-polygon [ <parameter class> <x0> <y0> <x1> <y1> ... <xn> <yn> ]``
 Pops expansion. Expands the polyon specified by the coordinates in the
-argument by the dimension popped from the stack.
+argument by the dimension popped from the stack. This expansion will
+work for any polygon, including concave shapes (which will become convex
+at the appropriate expansion). Note that some shapes produce interior
+holes at certain expansions, which may not be what is intended.
 
 Example program (from SMD rectangular padstack)
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