driver: watchdog: multiple doxygen improvements

- Fix incorrect struct fields documentation (`@param` can't be used)
- Removed redundant `@file` entry, public APIs may be split into
  multiple headers.
- Grouped and anchored options and flags, so they can be referenced.
- Guarded some internal fields with INTERNAL_HIDDEN.
- Exposed conditional struct fields, using __DOXYGEN__.
- Use `@see` in cases where used was instructed to look at other
  function calls.
- Added `errno` `@retval` to all functions. In case of external
  watchdogs, communication errors may be propagated.
- Other minor/typo fixes

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>