BTPROTO_L2CAP extenstion: Expand the docs & provide link targets for the constants

[encukou]

Hi! Here are my suggestions for the docs of python#129293. Merge this to update your PR.

• break out the tuple fields into a list, like AF_TIPC or AF_CAN
• define the referenced constants to avoid "missing target" warnings

(Ideally we'd merge allll the socket constant definitions with the usage documentation, so links go to informative text, but that's for a separate PR.)

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📚 Documentation preview 📚: https://cpython-previews--1.org.readthedocs.build/

[themadinventor]

Much better, thanks!