docs: Add operational note and fix step description for rebuild-storage

Note that graph-node must be stopped before running the command, making
explicit the single-operator assumption baked into the check-then-act
flow. Also correct the DROP SCHEMA step description to remove the
stale IF EXISTS qualifier.

Author: dimitrovmaksim

docs/graphman.md

@@ -467,6 +467,11 @@ The `chain rebuild-storage` command recovers from a situation where a chain's st
 (e.g. `chain42`) has been dropped or corrupted on the shard but the chain's metadata in
 `public.chains` still exists. This can happen after manual database operations or partial failures.
 
+> **Operational requirement:** Stop graph-node before running this command. The command
+> checks schema presence and then acts on that snapshot in a single transaction. Running
+> it while graph-node is active risks a concurrent schema modification between the check
+> and the transaction, which will cause the command to fail with an error.
+
 The command behaves differently depending on the state of the storage:
 
 **Storage missing** (non-destructive): the command silently rebuilds the schema and resets
@@ -477,7 +482,7 @@ existing schema and rebuilding it from scratch. Use `--force` to skip the prompt
 
 In both cases, the command performs the following steps in a single transaction:
 
-1. Drops the existing storage schema if present (`DROP SCHEMA IF EXISTS ... CASCADE`).
+1. Drops the existing storage schema if present (`DROP SCHEMA ... CASCADE`).
 2. Upserts the chain's row in `ethereum_networks` on the shard: inserts if missing, or repairs
    identity metadata (`namespace`, `net_version`, `genesis_block_hash`) and resets head tracking
    columns (`head_block_hash`, `head_block_number`, `head_block_cursor`) to `NULL` if the row exists.