Fix markup in a few weird places

[jorendorff]

In some places, non-bold text appears inside of <code>. Currently I don't think we render it correctly; the stylesheet makes all code elements bold.

For example: <code><i>constructor</i><b>.prototype</b></code>
would be better rendered as <code><var>constructor</var>.prototype</code>
with a stylesheet that includes code > var { font-weight: normal; }.

Similarly we have <code><b>1110</b><i>xxxx</i></code>.

There are also other places in the doc where non-bold Courier text is used for identifiers or bits of code. I tend to think that's just a historical thing and we don't need to preserve it.