Documentation: filesystems: mount_api: fix headings

Fix capitalization in two headings, correct one verb, and
demote one heading to a section heading.

Fixes: 791a17ee ("docs: filesystems: convert mount_api.txt to ReST")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/adaf123c-b394-f78c-53c0-671d7fda45e7@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>