docs(quinn): Clarify effects of setting AckFrequencyConfig

[gretchenfrage]

Per discussion in Discord [link], the actual extent to which setting ack_frequency_config to None disables the acknowledgement frequency extension is less than the documentation seems to imply. An endpoint with the ack frequency config set to None will still:

• advertise support for the extension by populating the min_ack_delay field of its transport parameters with Some
• process ACK_FREQUENCY and IMMEDIATE_ACK frames if it receives them
• send IMMEDIATE_ACK frames if the peer advertises support for the extension

The only thing it will not do is send the peer an ACK_FREQUENCY frame.

This PR updates the docs to reflect the behavior more closely.

[Ralith]

Thanks!