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: 16c01b2 ("doc/filesystems: more mount cleanups")
Fixes: 9cfccee ("[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>

Author: rddunlap

Documentation/filesystems/sharedsubtree.rst

@@ -147,6 +147,7 @@ replicas continue to be exactly same.
 
 
 3) Setting mount states
+-----------------------
 
 	The mount command (util-linux package) can be used to set mount
 	states::
@@ -612,6 +613,7 @@ replicas continue to be exactly same.
 
 
 6) Quiz
+-------
 
 	A. What is the result of the following command sequence?
 
@@ -673,6 +675,7 @@ replicas continue to be exactly same.
 		/mnt/1/test be?
 
 7) FAQ
+------
 
 	Q1. Why is bind mount needed? How is it different from symbolic links?
 		symbolic links can get stale if the destination mount gets
@@ -841,6 +844,7 @@ replicas continue to be exactly same.
 			     tmp  usr tmp usr tmp usr
 
 8) Implementation
+-----------------
 
 8A) Datastructure