mseal: update mseal.rst

Pedro Falcato's optimization [1] for checking sealed VMAs, which replaces
the can_modify_mm() function with an in-loop check, necessitates an update
to the mseal.rst documentation to reflect this change.

Furthermore, the document has received offline comments regarding the code
sample and suggestions for sentence clarification to enhance reader
comprehension.

[1] https://lore.kernel.org/linux-mm/20240817-mseal-depessimize-v3-0-d8d2e037df30@gmail.com/

Update doc after in-loop change: mprotect/madvise can have
partially updated and munmap is atomic.

Fix indentation and clarify some sections to improve readability.

Link: https://lkml.kernel.org/r/20241008040942.1478931-2-jeffxu@chromium.org


Fixes: df2a7df9 ("mm/munmap: replace can_modify_mm with can_modify_vma")
Fixes: 4a2dd02b ("mm/mprotect: replace can_modify_mm with can_modify_vma")
Fixes: 38075679 ("mm/mremap: replace can_modify_mm with can_modify_vma")
Fixes: 23c57d1f ("mseal: replace can_modify_mm_madv with a vma variant")
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Elliott Hughes <enh@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Stephen Röttger <sroettger@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Theo de Raadt" <deraadt@openbsd.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>