[Doc] Document special treatment of 1559 in provider and layers

[prestwich]

we need to write down somewhere that 1559 is somewhat privileged by the library in ways that other tx types and HF features are not. I.e. we generally assume that 1559 is active and proactively attempt to use it in gas estimation even tho we acknowledge that there may be chains that don't support it

Originally posted by @prestwich in https://github.com/alloy-rs/alloy/pull/363#discussion_r1535775698