document default interparagraph spacing

quad/quad/scribblings/quad.scrbl

@@ -454,7 +454,7 @@ The Q-expressions @racketresult['#,line-break], @racketresult['#,column-break],
 
 
 @defthing[para-break qexpr?]{
-The @tech{Q-expression} @racketresult['#,para-break]. Used to denote the start of a new paragraph.
+The @tech{Q-expression} @racketresult['#,para-break]. Used to denote the start of a new paragraph. The default @racket[space-before] a paragraph is 0; the default @racket[space-after] is 60% of the paragraph's @racket[line-height].
 }
 
 

quad/quadwriter/line.rkt

@@ -245,7 +245,9 @@
           new-lines
           (match q-after
             [(? column-break-quad? column-break) (list column-break)] ; hard column (or section or page) break
-            [#false (list (make-paragraph-spacer maybe-first-line :space-after (* default-line-height 0.6)))] ; paragraph break
+            [#false
+             (define default-paragraph-spacing (* default-line-height 0.6))
+             (list (make-paragraph-spacer maybe-first-line :space-after default-paragraph-spacing))] ; paragraph break
             [_ null]))) ; hard line break