[doc] Increase width of refguide content for readability (#3009)

The reference guide #content div has `max-width: 62.5em`. This is a bit
small for large screens in landscape mode. So, this commit increases the
width.

For landscape, in FHD window, ideally, `1920px` * `60%` +
`8em (div's padding)` = `80em` is a required width for readability. For
more wide window, the constant will be smaller than `60%` of the
window's width. Thus, `max-width` is desirable to be maximum of `80em`
and `60%` of a window.

For portrait (phone), the changes for landscape doesn't affect for
portrait because it changes `max-width` only.

So, this commit set `max-width` to `max(80em, 60%)`.

Fixes #2784.

docs/asciidoc/stylesheets/reactor.css

@@ -822,7 +822,7 @@ p a > code:hover {
     margin-right: auto;
     margin-top: 0;
     margin-bottom: 0;
-    max-width: 62.5em;
+    max-width: max(80em, 60%);
     *zoom: 1;
     position: relative;
     padding-left: 4em;