[quicwg/base-drafts] Persistent congestion period: text and pseudo-code don't agree (#3972)

[Jana Iyengar <[e-mail]>]

We resolved #3875 with #3889, but I realized that it has a problem. The text and the pseudo-code don't agree.

The text says:
```
The persistent congestion period SHOULD NOT start until there is at
least one RTT sample.
```
Since the `persistent congestion period` starts with a packet that was sent in the past, this statement means that persistent congestion only includes packets that were _sent_ after an RTT sample was taken.

The pseudo code says:
```
   InPersistentCongestion(largest_lost):
     // Persistent congestion cannot be declared on the
     // first RTT sample.
     if (is first RTT sample):
       return false
```
This says that the persistent congestion can only be _established_ on the second (or later) ack that yields an RTT sample. This allows for packets that were sent before the first RTT sample was taken to be included in the persistent congestion period.

I had convinced myself that the pseudo-code was adequate, but I missed the fact that the text was not aligned.

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