Documentation/filesystems: sharedsubtree: add section headings

Several of the sections are missing underlines. This makes the
generated contents have missing entries, so add the underlines.

Fixes: 16c01b20 ("doc/filesystems: more mount cleanups")
Fixes: 9cfcceea

 ("[PATCH] Complete description of shared subtrees.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230508055938.6550-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>