Don't keep shrinking font when nesting admonitions

In some places we nest admonitions, e.g. a note within a
release-version-specific part of docs. Each admonition has a font-size
of 85% of its parent element. Even for the first nested admonition
this results in a barely readable text.

Fix it so that only the first admonition shrinks font to 85% of
parent, and it doesn't shrink further when nesting.

Change-Id: I91992a29375eea1ca9971d05827a20153b1721a7

_custom/custom.css

@@ -37,6 +37,11 @@
   margin-bottom: 24px;
 }
 
+.admonition .admonition {
+    /* Don't keep shrinking the font for nested admonitions. */
+    font-size: 100%;
+}
+
 .admonition p {
   font-size: inherit;
 }