low-level api documentation cleanup

Various fixes to low-level API documentation:

* removed content-proxy
* removed obsolete or misleading wording (references to e10s, or "This module is not intended to be used directly by programs.")
* made sure deprecated modules point people at the replacements, where they exist
* added basic doc for tabs/utils
* changed various modules to use the <api></api> syntax, so they are formatted consistently and get tables of contents
* changed the text contents of tables of contents, to not include datatypes or arguments. This makes them less likely to wrap and therefore (I think) easier to read.
* some spelling, grammar, and formatting issues
* changed the way module H1 and titles are generated, so multi-part modules get called (for example) "tabs/utils", not just "utils"