docs(rpc): document the historical account state retention window

[devorun]

Summary

Addresses #2597.

GetAccount can return public account state at a past block, but the node only retains that history for the most recent 50 blocks (HISTORICAL_BLOCK_RETENTION, a compile-time store constant). Older blocks are rejected with INVALID_ARGUMENT ("block N has been pruned"). None of this was documented, so clients that anchor a flow to a reference block — e.g. multi-party signing — only discover the window by hitting the error.

This adds a "Historical State Retention" section to the RPC errors-and-limits page describing the window, how it is enforced, the error it produces, and that the boundary can lag the tip while readers still pin older snapshots.

Documentation only; no behavior change.

Changelog

[[entry]]
scope       = "docs"
impact      = "added"
description = "Document the historical account state retention window and the resulting GetAccount error"