docs: fix flag defaults and remove non-existent flags in fundamentals

[bloxster]

Summary

Fixes multiple inaccuracies across the fundamentals documentation section — stale defaults, non-existent flags, and incorrect port labels.

Changes by file

fundamentals/basic-usage.md

• --torrent.download.rate: corrected default from 128mb → 512mb; reworded description to reflect the flag caps bandwidth rather than overclock it

fundamentals/performance-tricks.md

• Same download rate default fix: 128MB → 512mb

fundamentals/default-ports.md

• Port 30304 (sentry) labelled eth/67 → eth/69

fundamentals/logs.md

• --verbosity default was shown as 2 (numeric level); corrected to info (the actual string value used by the flag)

fundamentals/security.md

• Removed mention of --txpool.nolocals=true — this flag does not exist in Erigon

fundamentals/multiple-instances.md

• Removed --db.growth.step from example code block — this flag does not exist in Erigon
• Fixed env var: ERIGON_SNAPSHOT_MADV_RND → SNAPSHOT_MADV_RND (appears twice)

fundamentals/modules/rpc-daemon.md

• Removed --grpc from the Erigon command in the Remote mode table — the flag is not available on the main erigon binary
• Added a clarifying note explaining that the internal gRPC interface is exposed automatically via --private.api.addr

fundamentals/modules/downloader.md

• --torrent.download.rate default: "128mb" → "512mb"
• --torrent.upload.rate default: "4mb" → "16mb"
• --torrent.download.slots: marked as DEPRECATED, default corrected 128 → 32

fundamentals/configuring-erigon/README.md

• --discovery.v4 default: true → false
• Removed --discovery.parallelism (non-existent flag)
• Removed --discovery.ban-threshold (non-existent flag, also had a markdown typo)
• --log.dir.verbosity default: info → dbug
• Consolidated two duplicate commitment-history flags into one combined entry
• Added --override.amsterdam to General Options (narrative + help listing)
• Added MCP Server section: --mcp.addr, --mcp.port (narrative + help listing)
• Added FCU section: --fcu.timeout, --fcu.background.prune, --fcu.background.commit (narrative + help listing)