tailcfg: document use of CapMap for peers

Updates tailscale/corp#17516
Updates #11508

Change-Id: Iad2dafb38ffb9948bc2f3dfaf9c268f7d772cf56
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f