Style tweak for API rendering (#1012)

Tighter bound on the `max-width` property for paragraphs inside rendered
APIs. AFAICT it's only meant to apply to the summary.

assets/scss/custom.scss

@@ -267,10 +267,13 @@ footer {
   margin: 1rem 0 3rem 0;
 
   details {
-
     summary {
       padding: .5rem 0;
       list-style-position: outside;
+
+      p {
+        max-width: 80%;
+      }
     }
   }
 
@@ -313,10 +316,6 @@ footer {
     margin-bottom: 1.5rem;
   }
 
-  p {
-    max-width: 80%;
-  }
-
   p code, table code {
     background-color: $white;
   }