Update semantics documentation to describe changes as part of the open-after-close fix (#1752)

Update semantics documentation to describe changes done as part of the
open-after-close fix in [PR
#1704](#1704)

Updates done to `doc/SEMANTICS.md` documentation and some more details
added in`mountpoint-s3/CHANGELOG.md`.

### Does this change impact existing behavior?

No, documentation update only.

### Does this change need a changelog entry? Does it require a version
change?

No, documentation update only.

---

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and I agree to the terms of
the [Developer Certificate of Origin
(DCO)](https://developercertificate.org/).

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Signed-off-by: Mansi Pandey <mansipnd@amazon.com>
Co-authored-by: Mansi Pandey <mansipnd@amazon.com>

Author: mansi153