SDL_filesystem.h: Fill in missing documentation.

include/SDL3/SDL_filesystem.h

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 typedef enum SDL_Folder
 {
     SDL_FOLDER_HOME,        /**< The folder which contains all of the current user's data, preferences, and documents. It usually contains most of the other folders. If a requested folder does not exist, the home folder can be considered a safe fallback to store a user's documents. */
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     SDL_FOLDER_DESKTOP,     /**< The folder of files that are displayed on the desktop. Note that the existence of a desktop folder does not guarantee that the system does show icons on its desktop; certain GNU/Linux distros with a graphical environment may not have desktop icons. */
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     SDL_FOLDER_DOCUMENTS,   /**< User document files, possibly application-specific. This is a good place to save a user's projects. */
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     SDL_FOLDER_DOWNLOADS,   /**< Standard folder for user files downloaded from the internet. */
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     SDL_FOLDER_MUSIC,       /**< Music files that can be played using a standard music player (mp3, ogg...). */
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     SDL_FOLDER_PICTURES,    /**< Image files that can be displayed using a standard viewer (png, jpg...). */
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     SDL_FOLDER_PUBLICSHARE, /**< Files that are meant to be shared with other users on the same computer. */
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     SDL_FOLDER_SAVEDGAMES,  /**< Save files for games. */
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     SDL_FOLDER_SCREENSHOTS, /**< Application screenshots. */
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     SDL_FOLDER_TEMPLATES,   /**< Template files to be used when the user requests the desktop environment to create a new file in a certain folder, such as "New Text File.txt".  Any file in the Templates folder can be used as a starting point for a new file. */
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     SDL_FOLDER_VIDEOS,      /**< Video files that can be played using a standard video player (mp4, webm...). */
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     SDL_FOLDER_COUNT        /**< Total number of types in this enum, not a folder type by itself. */
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 } SDL_Folder;
 
 /**
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 /* Abstract filesystem interface */
 
+/**
+ * Types of filesystem entries.
+ *
+ * Note that there may be other sorts of items on a filesystem: devices,
+ * symlinks, named pipes, etc. They are currently reported as
+ * SDL_PATHTYPE_OTHER.
+ *
+ * \since This enum is available since SDL 3.1.3.
+ *
+ * \sa SDL_PathInfo
+ */
 typedef enum SDL_PathType
 {
     SDL_PATHTYPE_NONE,      /**< path does not exist */
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     SDL_PATHTYPE_OTHER      /**< something completely different like a device node (not a symlink, those are always followed) */
 } SDL_PathType;
 
+/**
+ * Information about a path on the filesystem.
+ *
+ * \since This datatype is available since SDL 3.1.3.
+ *
+ * \sa SDL_GetPathInfo
+ * \sa SDL_GetStoragePathInfo
+ */
 typedef struct SDL_PathInfo
 {
     SDL_PathType type;      /**< the path type */
···
 } SDL_PathInfo;
 
 /**
- * Flags for path matching
+ * Flags for path matching.
  *
  * \since This datatype is available since SDL 3.1.3.
  *