More syntax changes with code blocks in mli/mld files

[jonludlam]

odoc-parser now parses the tags in code blocks. The syntax is:

{@ocaml tag "tag two" key=value "key two"="value two" [
...code...
]}

This is not compatible with the syntax used by mdx, where we have comma-separated tags or relations. One easy fix is that we could simply have mdx's labels as a key-value pair:

{@ocaml mdx-labels="skip,ocaml,version<=4.06,os_type<>Win32" [
...
]}

Thoughts?