docs: clarify --grpc flag availability in grpc.md

[bloxster]

Summary

Fixes two misleading statements in docs/gitbook/src/interacting-with-erigon/grpc.md about the --grpc flag:

• Introduction paragraph: The original text said the gRPC server "must be explicitly enabled using --grpc" when starting the RPC daemon, without clarifying that this flag only applies to the standalone rpcdaemon binary. The main erigon binary does not accept --grpc; its internal gRPC services (txpool, downloader, sentry, etc.) start automatically and are accessed via --private.api.addr. Reworded to make this distinction clear.

• Availability section: The bullet point "gRPC services are available when enabled with the --grpc flag" now explicitly notes it is the standalone rpcdaemon binary that accepts this flag.

Files changed

• docs/gitbook/src/interacting-with-erigon/grpc.md