doxygen: fixed duplicated comments for scheduler APIs

Similar to PR #10674, `src/scheduler_mp.c` and
`src/scheduler_up.c` also have similar issues.

After investigation, we found: in commit 2c98ce4
"[HUST CSE][document] Fix some comments, invalid
grouping commands and warnings in Doxygen." This patch
excluded `src/scheduler_mp.c` and `src/scheduler_up.c`
from the Doxygen build and include
`documentation/doxygen/thread.h` (the new file path
after adjustment is `documentation/0.doxygen/core/thread.h`),
placing the Doxygen comments for some functions in this file.

However, this approach has its problems:

- The comments in `documentation/0.doxygen/core/thread.h`
  are separated from the source code, making them
  difficult to maintain. This also results in the
  comments for the newly added functions in
  `src/scheduler_mp.c` and `src/scheduler_up.c` not
  being updated in a timely manner and failing to
  be compiled into Doxygen (becoming HTML).

- For the average programmer, it's unlikely that
  they'll consider the `documentation/0.doxygen/`
  directory for function declarations and comments,
  let alone the need to maintain the Doxygen comments
  in that directory. Currently, the header files under
  `documentation/0.doxygen/` should strictly be used
  only to define Doxygen groups and should not include
  Doxygen comments for the code itself. (Similar
  issues exist in documentation/0.doxygen/3.hardware.h
  and documentation/0.doxygen/core/systeminit.h, which
  require further cleanup.)

The solution is similar to PR #10674, where only MP
documentation is generated by default. For functions
defined in MP but not in UP, we use the "@note"
directive in the function's doxygen comment to indicate
whether the function is supported in both UP and MP,
or only in MP.

After implementing this solution, the file
`documentation/0.doxygen/core/thread.h` can be deleted.

Signed-off-by: Chen Wang <[email protected]>

Author: unicornx