wikiheaders: bridge wiki Category docs to the headers!

Did an initial cleanup on the headers and wrote a few pieces of documentation,
but this needs more work to fill out the documentation.

include/SDL3/SDL_assert.h

@@ -20,9 +20,39 @@
 */
 
 /**
- *  \file SDL_assert.h
+ * # CategoryAssert
  *
- *  Header file for assertion SDL API functions
+ * A helpful assertion macro!
+ *
+ * SDL assertions operate like your usual `assert` macro, but with some added
+ * features:
+ *
+ * - It uses a trick with the `sizeof` operator, so disabled assertions
+ *   vaporize out of the compiled code, but variables only referenced in the
+ *   assertion won't trigger compiler warnings about being unused.
+ * - It is safe to use with a dangling-else: `if (x) SDL_assert(y); else
+ *   do_something();`
+ * - It works the same everywhere, instead of counting on various platforms'
+ *   compiler and C runtime to behave.
+ * - It provides multiple levels of assertion (SDL_assert, SDL_assert_release,
+ *   SDL_assert_paranoid) instead of a single all-or-nothing option.
+ * - It offers a variety of responses when an assertion fails (retry, trigger
+ *   the debugger, abort the program, ignore the failure once, ignore it for
+ *   the rest of the program's run).
+ * - It tries to show the user a dialog by default, if possible, but the app
+ *   can provide a callback to handle assertion failures however they like.
+ * - It lets failed assertions be retried. Perhaps you had a network failure
+ *   and just want to retry the test after plugging your network cable back
+ *   in? You can.
+ * - It lets the user ignore an assertion failure, if there's a harmless
+ *   problem that one can continue past.
+ * - It lets the user mark an assertion as ignored for the rest of the
+ *   program's run; if there's a harmless problem that keeps popping up.
+ * - It provides statistics and data on all failed assertions to the app.
+ * - It allows the default assertion handler to be controlled with environment
+ *   variables, in case an automated script needs to control it.
+ *
+ * To use it: do a debug build and just sprinkle around tests check your code!
  */
 
 #ifndef SDL_assert_h_