docs: mark the EEPROM claim as inference, not specification (#392)

The WIT guide asserted 'All other writable VPP registers ARE written to EEPROM'
as though Growatt had said so. They have not. The only statement in any document
we hold is the 'Not storage' annotation on 30101, 30154 and 30407-30409; the
claim about everything else is our reasoning from that annotation's existence.

A reporter building a price-driven TOU controller read our docs carefully, could
not find the spec behind the claim, and asked whether one existed. It does not,
and the wording should not have implied it did.

The advice is unchanged - write infrequently to anything not marked 'Not storage'
- but it is now labelled as caution rather than a documented limit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: 0xAHA

docs/controls/wit-guide.md

@@ -79,7 +79,15 @@ model rejects the write, use the inverter display panel or the manufacturer app
 
 ### Remote Power Control (Registers 30407-30409)
 
-> **EEPROM-safe:** Registers 30407, 30408, and 30409 are explicitly marked **"Not storage"** in the VPP V2.03 spec — they bypass non-volatile memory entirely. High-frequency writes (e.g., updating charge/discharge power every minute based on spot prices or live consumption) do not risk wearing out the inverter's flash memory. All other writable VPP registers ARE written to EEPROM and should not be written at high frequency.
+> **EEPROM-safe:** Registers 30407, 30408, and 30409 are explicitly marked **"Not storage"** in the VPP V2.03 spec — they bypass non-volatile memory entirely. High-frequency writes (e.g., updating charge/discharge power every minute based on spot prices or live consumption) do not risk wearing out the inverter's flash memory.
+>
+> **About the other registers — this is an inference, not a specification.** Growatt marks
+> these three as "Not storage" and says nothing about the rest, so we assume the unmarked
+> ones *are* written to non-volatile memory and treat them conservatively. Growatt has not
+> stated that, we have not measured it, and the protocol documents carry no memory or
+> endurance column for the V1.39 ranges at all. Write infrequently to anything not marked
+> "Not storage", but understand that as caution rather than as a documented limit — see
+> [#392](https://github.com/0xAHA/Growatt_ModbusTCP/issues/392).
 
 #### Register 30407: Remote Power Control Enable
 - 0 = Disabled