RFR: JDK-8204330: Javadoc IllegalArgumentException: HTML special chars in constant value

[Jonathan Gibbons]

Please review a simple fix for an issue that caused 
IllegalArgumentException when processing a constant value containing the 
special HTML characters, < & >.

In HTMLWriter, the fix is to simply remove the inappropriate restriction 
on the presence of the special HTML chars.  The existing use of 
StringContent will handle the characters correctly, by suitably encoding 
them.

In TagletWriter, the inappropriate use of RawHtml is replaced by the use 
of StringContent.

A new test case is added to the existing test for the {@value} tag. It 
verifies the correct encoding of the characters, and verifies the 
absence of unencoded characters and the absence of any exceptions.

JBS: https://bugs.openjdk.java.net/browse/JDK-8204330
Webrev: http://cr.openjdk.java.net/~jjg/8204330/webrev.00/index.html

-- Jon