Add 'CoAP' to doc-valid-idents

[chrysn]

CoAP is a name of a network protocol common in embedded systems; one would talk in documentation about "a CoAP server" or "a CoAP client" without referring to a specific type.

This PR fixes false positives that arise from that use.

changelog: [doc_markdown]: Add CoAP to doc-valid-idents.

As this review is identical in structure to #13460, I'm asking for a the same reviewer (if that works):

r? @Centri3

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[Centri3]

This seems right in the same ballpark as IPv4 and IPv6 even if less common. I think we could always add stuff like https in the future

@bors r+

[bors]

📌 Commit bd4aa17 has been approved by Centri3

It is now in the queue for this repository.

[bors]

⌛ Testing commit bd4aa17 with merge 1998abc...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Centri3
Pushing 1998abc to master...