shmem: update documentation

shmem: update documentation

Update the docs to reflect a bit better why some folks prefer tmpfs over
ramfs and clarify a bit more about the difference between brd ramdisks.

While at it, add THP docs for tmpfs, both the mount options and the sysfs
file.

Link: https://lkml.kernel.org/r/20230309230545.2930737-6-mcgrof@kernel.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>