Improve WisunInterface set_network_size docs

[artokin]

Summary of changes

Clarify set_network_size usage and possible parameter values.
Clarify wisun-network-size help text in mbed-mesh-api/mbed_lib.json

This change is copied from: #14431

Impact of changes

Migration actions required

Documentation

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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

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[ciarmcom]

@artokin, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

Pull request has been modified.

[adbridge]

@0xc0170 are you happy with the updates ?

[adbridge]

I'll kick off the Ci in the bg pending Martin's approval on his return from holiday.

[adbridge]

Ci started

[mbed-ci]

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