Release 0.21.0: CHANGELOG wording (SAFETY-136, SAFETY-138 known issues)

SAFETY-136: the line said 10 to 15 s late is not caught, then that 15 s late
is caught after about 4 minutes. It now says 10 s is not caught and 15 s only
after about 4 minutes, as the card says.
SAFETY-138: the 30 s timer that zeroes a late write is the ESPHome board's
watchdog (README), not the inverter's own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

goodwe_controller/CHANGELOG.md

@@ -100,13 +100,13 @@ Contents of this release, in merge order since 0.20.1 (`d53f70f`):
 - In the last minute of a 15-minute period, with Home Assistant readings 30 s late, the period can end about 80 W over the peak aim and over the limit: 3 530 W against a 3 450 W aim and a 3 500 W limit in a rig test with a 2 kW load added 45 s before the end. The peak guard sees the load too late to hold it back ([[SAFETY-133]]).
 - With Home Assistant readings 10 s late, the grid charge in a cheap run can swing by several kW. In a test the add-on asked for an 8.7 kW charge (9.6 kW on 0.20.0) ([[SAFETY-132]]).
 - A plan rebuilt while the battery levels are missing for a moment can drop a cheap run that has not started yet, so the battery does not charge that day. Seen in a replay of real prices, where it cost about EUR 0.40 of extra grid use; the Leftover solar change in 0.20.1 no longer triggers it in those replays, but the cause stays ([[OPT-37]]).
-- On a 5 s meter, Home Assistant readings 10 to 15 s late are not caught. With a changing load such as an oven or a kettle, 21 to 142 Wh can go to the grid in a simulation, against 2 to 27 Wh with readings on time. At 15 s late they are caught only after about 4 minutes ([[SAFETY-136]]).
+- On a 5 s meter, Home Assistant readings 10 s late are not caught, and readings 15 s late only after about 4 minutes. With a changing load such as an oven or a kettle, 21 to 142 Wh can go to the grid in a simulation, against 2 to 27 Wh with readings on time ([[SAFETY-136]]).
 - With readings 10 to 15 s late, the hold can end in the middle of the delay and the swing can come back. Seen in rig tests with a 2 s and a 5 s meter ([[SAFETY-134]]).
 - With readings 10 s late on a 1 s meter, the grid can still move between -422 and +340 W while the batteries are held. With a very long delay (30 to 90 s) the request can swing by about 900 W every few seconds. Seen in review tests ([[SAFETY-130]]).
 - A battery that answers very slowly (10 s or more) can be taken for late Home Assistant readings and held, and the page then blames Home Assistant. A GoodWe battery settles in about 6 s and this has not been seen on one ([[SAFETY-131]]).
 - The page row for late readings reads awkwardly in English ("Home Assistant readings show requests about N seconds late, so control holds the batteries"), and the Dutch text misses a word ([[UX-78]]).
 - On a night grid charge held to the peak limit, the peak guard can stop the charge 1 to 3 s into a 15-minute period and keep it paused for that whole period. In a rig test this cost up to about 0.27 kWh of grid charge per run; it may come from the sped-up rig clock ([[SAFETY-135]]).
-- When the add-on stops, a write that takes more than 2 s can still land after the 0 W, and the inverter's own 30 s timer then zeroes it. After the Stop button, a cycle that had already decided can send its share (560 to 670 W) just after the 0 W; the next cycle sends 0 W again ([[SAFETY-138]]).
+- When the add-on stops, a write that takes more than 2 s can still land after the 0 W, and the 30 s safety timer on the ESPHome board then sets 0 W. After the Stop button, a cycle that had already decided can send its share (560 to 670 W) just after the 0 W; the next cycle sends 0 W again ([[SAFETY-138]]).
 - In the last hour of a cheap run, Home can draw a battery's grid charge up to an hour past the time control stops charging it. It only draws too much, never too little ([[UX-79]]).
 - A tomorrow solar forecast sensor that reports just after midnight with yesterday's value can be used as the new day's forecast for up to 12 hours. This has not been seen on a site ([[OPT-39]]).
 - During a cheap-hours charge, at some 15-minute boundaries the add-on sends 0 W for one tick ("no trade (headroom)") and then goes on. Seen in a rig test, in 5 of 8 cases; once, with a frozen battery reading, it left a battery at 0 W until 23:45 ([[OPT-40]]).