XArray: minor documentation improvements

- Replace "they" with "you" where "you" is used in the preceding
  sentence fragment.
- Mention `xa_erase` in discussion of multi-index entries.  Split this
  into a separate sentence.
- Add "call" parentheses on "xa_store" for consistency and
  linkification.
- Add caveat that `xa_store` and `xa_erase` are not equivalent in the
  presence of `XA_FLAGS_ALLOC`.

Link: https://lkml.kernel.org/r/20241105-xarray-documentation-v5-1-8e1702321b41@gmail.com


Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>