Add DTLS in Connectivity's design document

[michalpasztamobica]

Summary of changes

This is an outcome of a discussion in a recent PR targeting other aspects of this design document. DTLSSocket was added to mbed-os after this document was created.

Impact of changes

None

Migration actions required

None

Documentation

None

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@kivaisan
@AnttiKauppila
@adbridge
@AnotherButler

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[ciarmcom]

@michalpasztamobica, thank you for your changes.
@AnotherButler @kivaisan @AnttiKauppila @adbridge @ARMmbed/mbed-os-connectivity @ARMmbed/mbed-os-maintainers please review.

[bulislaw]

LGTM, @ARMmbed/mbed-os-ipcore please review