docs: add FreeRTOS API docs

docs: add FreeRTOS API docs

- Use `code` tags instead of a mix of `<pre></pre>` and
  `@verbatim .. @endverbatim`
- Remove manually added function prototypes from comment blocks
- Remove of grouping (`\defgroup`) — some extra work is needed
  to make groups compatible with the way we auto-generate API
  reference from Doxygen XML files. It's pretty easy to add the
  grouping directives back if/when we implement support for
  Doxygen groups in the later stages of documentation build
  process.
- Hide private APIs under `@cond .. @endcond`
- Convert some comments into Doxygen-compatible ones
- Fix various documentation issues: missing documentation for
  some parameters, mismatch between parameter names in comment
  block and in function prototype.
- Add doxygen comments for functions which didn't have them
  (thread local storage).
- Add [out] param tags where necessary
- Redefine `xTaskCreate` and `xTaskCreateStatic` as inline
  functions instead of macros.