Write documentation for each wiki Category.

[icculus]

This is the only still-pending thing worth doing from #9440 that should really be in the 3.2.0 milestone, so I'm putting it in its own issue.

Most of these are written already, and I believe to be high-quality. The ones that aren't are basically one-liners like SDL_power.h:

/**
 * # CategoryPower
 *
 * SDL power management routines.
 */

These would benefit from a more in-depth idea of what's offered and how to use it.

For 3.2.0, we at least need to make sure we know which of these need work, which should only take a few minutes. Writing them out in detail should take an hour or two, I suspect, but they don't have to be the Great American Novel if they are at least not embarrassing, for 3.2.0, and then improvements can bump to 3.4.0 or whatever.

[icculus]

I filled in a few things real fast in 7f9ee31, but these are the ones that are still needed:

• SDL_messagebox.h
• SDL_mouse.h
• SDL_mutex.h
• SDL_power.h
• SDL_sensor.h
• SDL_thread.h
• SDL_time.h
• SDL_timer.h
• SDL_touch.h
• SDL_tray.h

We can live with what's there, but what's there in all of these is one-liners like "SDL power management routines."

If I can find an hour for this, I can finish for 3.2.0, but this can bump to 3.4.0 otherwise.

[icculus]

Some of these could be more detailed, and the SDL_sensor.h category docs are downright anemic, but I'm willing to call all this Good Enough for 3.2.0 now.