add metadata topics
minor fixes
edit metadata topic
edit
edit metadata topics

more x-refs, code highlighting for xml, properly indented code blocks,
various other adjustments (including rephrasing some sections)
Also add a "seealso" block to the symbols doc
Address feedback from 7bf1ec38 (#102)
"document" -> "page"
Very minor adjustments

From ccb7f2b feedback.
Merge pull request #102 from guillermooo/new-topic-metadata

comments, symbols, metadata