documentation: fix doxygen for `pthread_*.h`

The pthread header files aren't in the doxygen page anymore after #1137,
because I `@file`'d the `.c` files, not the `.h` files.

This change moves doxygen boilerplate.

Closes #1199.
Merge pull request #1208 from Kijewski/issue-1199

documentation: fix doxygen for `pthread_*.h`