sphinx.rst: Add note about code snippets embedded in the text

There's a paragraph that explains how to create fixed width text block,
but it doesn't explains how to create fixed width text inline, although
this feature is really used through the documentation. Fix that adding a
quick note about it.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>