[e] (0) Clarify that removing src='' has no effect.

Fixing http://www.w3.org/Bugs/Public/show_bug.cgi?id=7632

git-svn-id: http://svn.whatwg.org/webapps@3925 340c8d12-0b0e-0410-8428-c7bf67bfef74

index

  <code title=dom-media-NETWORK_EMPTY><a href=#dom-media-network_empty>NETWORK_EMPTY</a></code> is set or
  changed, the user agent must invoke the <a href=#media-element>media element</a>'s
  <a href=#concept-media-load-algorithm title=concept-media-load-algorithm>resource selection
  algorithm</a>.</p>
  algorithm</a>. (<em>Removing</em> the <code title=attr-media-src><a href=#attr-media-src>src</a></code> attribute does not do this, even
  if there are <code><a href=#the-source-element>source</a></code> elements present.)</p>

  <p>The <dfn id=dom-media-src title=dom-media-src><code>src</code></dfn> IDL
  attribute on <a href=#media-element title="media element">media elements</a> must

source

  <code title="dom-media-NETWORK_EMPTY">NETWORK_EMPTY</code> is set or
  changed, the user agent must invoke the <span>media element</span>'s
  <span title="concept-media-load-algorithm">resource selection
  algorithm</span>.</p>
  algorithm</span>. (<em>Removing</em> the <code
  title="attr-media-src">src</code> attribute does not do this, even
  if there are <code>source</code> elements present.)</p>

  <p>The <dfn title="dom-media-src"><code>src</code></dfn> IDL
  attribute on <span title="media element">media elements</span> must