include: Fill in more category documentation.

Reference Issue #11874.

include/SDL3/SDL_filesystem.h

@@ -22,7 +22,23 @@
 /**
  * # CategoryFilesystem
  *
- * SDL Filesystem API.
+ * SDL offers an API for examining and manipulating the system's filesystem.
+ * This covers most things one would need to do with directories, except for
+ * actual file I/O (which is covered by [CategoryIOStream](CategoryIOStream)
+ * and [CategoryAsyncIO](CategoryAsyncIO) instead).
+ *
+ * There are functions to answer necessary path questions:
+ *
+ * - Where is my app's data? SDL_GetBasePath().
+ * - Where can I safely write files? SDL_GetPrefPath().
+ * - Where are paths like Downloads, Desktop, Music? SDL_GetUserFolder().
+ * - What is this thing at this location? SDL_GetPathInfo().
+ * - What items live in this folder? SDL_EnumerateDirectory().
+ * - What items live in this folder by wildcard? SDL_GlobDirectory().
+ * - What is my current working directory? SDL_GetCurrentDirectory().
+ *
+ * SDL also offers functions to manipulate the directory tree: renaming,
+ * removing, copying files.
  */
 
 #ifndef SDL_filesystem_h_