Correct the streaming limit docs and stale SDK and proto comments

[bkeroack]

Problem

Some streaming-API docs and comments describe behaviour the server does not have.

1. gRPC limits table. The operator-limits tables in docs/api/streaming.md §12 and the manual describe eventsgrpcmaxsubscriptions as the "watch-set size per gRPC stream". It caps concurrent Subscribe and Watch streams across all connections (events/src/grpc.rs, shared active_subs counter). The gRPC carrier has no general per-stream entry cap, only the per-connection silent-payment and descriptor caps. The same tables call eventsgrpcmaxconns "concurrent gRPC streams"; it caps TCP connections. The config reference and the GrpcLimits doc comments named only Subscribe.
2. SetCursor rate limit. Both SDKs' docs for the reserved RateLimited error kind say the server silently drops an over-rate SetCursor. Since feat(events): deterministic mid-stream SetCursor re-anchor ack/reject (#439) #441 it answers in-band with CursorRejected (RATE_LIMITED).
3. Proto placeholders. Two comments in events.proto still say silent-payment matching (AddSilentPayments) and BlockTweaks emit and replay "land in a later change". Both have shipped.

The docs also promise that an over-quota watch add is rejected with RESOURCE_EXHAUSTED / 429, while the server drops it without a signal. That one is fixed in the node by #899, which makes the server report the rejection in-band, so this PR leaves that text alone.

Fix

Documentation and comments only; no behaviour changes.

• docs/api/streaming.md §12, docs/manual/src/streaming.md, config-reference.md: correct the two limit rows.
• events/src/grpc.rs: the cap's doc comments name Watch as well as Subscribe.
• satd-events-client/src/error.rs (RateLimited) and clients/go/errors.go (KindRateLimited): say how the server reports an over-rate SetCursor.
• events.proto: drop the two placeholder sentences. clients/go/eventspb is regenerated with clients/go/gen.sh; the only diff is those two comments.

Ran locally: clients/go/lint.sh (gofmt, vet, staticcheck, errcheck), clients/go/gen.sh (bindings match the proto), mdbook build docs/manual, and RUSTDOCFLAGS=-D warnings cargo doc -p satd-events-client --no-deps. The new intra-doc links resolve. That command still fails on 8 unresolved links and 1 private-item link that are already on the base branch, in event.rs, lib.rs and resilient_watch.rs; CI does not run it.

Notes

• Targets release/0.6, for 0.6.1, with Report a refused watch add in-band (WatchAddRejected) #899, so 0.6.1 ships the corrected docs and comments. It was first opened against master; the commit forked at the release/0.6 cut and replays with the same patch. A forward-port to master follows once it merges; the published manual builds from master, so its fix goes live then.

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