[boost] Change to documentation writing guidelines

Hi, I'd like to change the guideline for writing reference documentation to include header and namespace information in the detailed documentation. If you arrive at a page by a link or from a search engine, it isn't always easy to work this out. I'll add them to the list here: http://beta.boost.local/doc/libs/release/more/writingdoc/structure.html#deta... Boostbook documentation already includes this. Any objections? Daniel