#3232 Docs: serve token GC per-sweep limit and quoting 0 in serve.yaml

From swamp-club#2535. The manual page content/manual/reference/swamp-serve/serve-flags.md needs two updates once #2535 ships:

1. Token garbage collection section: a sweep now collects at most 100 tokens (counting tokens that were collected or failed). A larger backlog, for example every revoked or expired token on a long-running deployment after an upgrade, drains over later sweeps. Tokens that failed in the previous sweep are tried last. Revoked and expired tokens are refused at authentication either way, so the delay only keeps them in swamp access token list longer.

2. Config file table: duration values such as 0 for token-gc-interval and token-gc-grace-period must be quoted in serve.yaml (token-gc-interval: '0' with double quotes). An unquoted number fails with: expected string, got number. After #2535 the error also suggests quoting the value.