Merge multiple comment tokens into a single one.

The tokens T_COMMENT_START, T_TEXT and T_COMMENT_END have been merged into a
single token: T_COMMENT. This simplifies both the lexer and the parser.
Merge multiple CDATA tokens into a single token.

The tokens T_CDATA_START, T_TEXT and T_CDATA_END have been merged together into
T_CDATA.
Cleaned up the XML lexer documentation.