Member access documentation is out of date

[geoffromer]

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Description of the bug:

/docs/design/expressions/member_access.md describes member access as a built-in operation, but as of #3720, member access is mediated by extensible interfaces such as BindToValue and BindToReference.

Link to documentation:

https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/member_access.md

What should it say instead?

See generally https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3720.md

Any other information, logs, or outputs that you want to share?

No response