extra/cap: add a summary and short docs to cap vocab

extra/cap/cap-docs.factor

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+USING: cap help.markup help.syntax images opengl ui.gadgets.worlds ;
+IN: cap
+
+HELP: screenshot.
+  { $values { "window" world } }
+  { $description
+    "Opens a window with a screenshot of the currently active window."
+  } ;
+
+HELP: screenshot
+  { $values { "window" world } { "bitmap" image } }
+  { $description
+    "Creates a bitmap image of a UI window."
+  }
+  { $notes "If the current " { $link gl-scale-factor } " is " { $snippet "2.0" } ", then the " { $snippet "2x" } " slot in the resulting " { $link image } " will be " { $link t } "." } ;

extra/cap/summary.txt

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+Creating and displaying screenshots of Factor