docs: Transition remaining DocBook XML to markdown/HTML

These remaining DocBook tags are mostly harmless; they are passed as
HTML5 tags to the browser, which effectively treats them as just
<span> elements, so all this mistake is doing is dropping some
styling.

This lack of of styling can be seen at:

 - https://docs.gtk.org/gio/enum.DriveStartStopType.html "ATA SECURITY
   UNLOCK DEVICE" is not indicated to be a quote or anything.

 - https://docs.gtk.org/gio/struct.UnixMountEntry.html "/media/cdrom"
   is not rendered in monospace.

 - https://docs.gtk.org/gio/struct.UnixMountPoint.html "/dev" is not
   rendered in monospace.

 - https://docs.gtk.org/glib/type_func.Thread.init.html the notes are
   not indicated to be anything other than regular paragraphs.

Fix that.

gio/gioenums.h

@@ -290,8 +290,8 @@ typedef enum /*< flags >*/ {
  *    assemble/disassemble a virtual drive from several physical
  *    drives.
  * @G_DRIVE_START_STOP_TYPE_PASSWORD: The start/stop methods will
- *    unlock/lock the disk (for example using the ATA <quote>SECURITY
- *    UNLOCK DEVICE</quote> command)
+ *    unlock/lock the disk (for example using the ATA `SECURITY UNLOCK
+ *    DEVICE` command)
  *
  * Enumeration describing how a drive can be started/stopped.
  *

gio/gunixmounts.h

@@ -30,7 +30,7 @@ G_BEGIN_DECLS
 /**
  * GUnixMountEntry:
  *
- * Defines a Unix mount entry (e.g. <filename>/media/cdrom</filename>).
+ * Defines a Unix mount entry (e.g. `/media/cdrom`).
  * This corresponds roughly to a mtab entry.
  **/
 typedef struct _GUnixMountEntry GUnixMountEntry;
@@ -42,7 +42,7 @@ GType g_unix_mount_entry_get_type (void) G_GNUC_CONST;
 /**
  * GUnixMountPoint:
  *
- * Defines a Unix mount point (e.g. <filename>/dev</filename>).
+ * Defines a Unix mount point (e.g. `/dev`).
  * This corresponds roughly to a fstab entry.
  **/
 typedef struct _GUnixMountPoint GUnixMountPoint;