client/local, ipn, tailcfg: document which LocalAPI client APIs are stable

The client/local package doc said its API is not necessarily stable, but
that caveat was easy to miss and only a few cert methods said anything
explicit either way. People have been surprised by IPN bus changes
between releases.

Add explicit "API maturity" notes, matching the existing wording on the
cert methods, marking stable: BugReport, BugReportWithOpts, CertDomains,
CheckUpdate, CurrentDERPMap, DialTCP, UserDial, DisconnectControl,
GetPrefs, EditPrefs, Status, StatusWithoutPeers, SetUseExitNode,
SwitchProfile, UserProfile, and the WhoIs* methods. Mark unstable:
ipn.Notify, WatchIPNBus, DoLocalRequest, the Debug*, Drive*, Check*,
EventBus*, and Stream* methods, SetComponentDebugLogging,
TailDaemonLogs, ShutdownTailscaled, GetDNSOSConfig, GetEffectivePolicy,
GetServeConfig, and GetAppConnectorRouteInfo.

Also note on tailcfg.DERPMap that the type is subject to minor changes
over time though its general shape is stable, document that
ipn.Prefs.CorpDNS is the internal name for "tailscale set --accept-dns",
and add a package doc paragraph to client/local saying that methods
without an explicit API maturity note should be assumed unstable.

Updates #20406

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I9333c58ae312e392c61d7de77987282e84ce2aeb

client/local/debugportmapper.go

@@ -50,6 +50,9 @@ type DebugPortmapOpts struct {
 // process.
 //
 // opts can be nil; if so, default values will be used.
+//
+// API maturity: this method is not considered a stable API and is
+// subject to change between releases.
 func (lc *Client) DebugPortmap(ctx context.Context, opts *DebugPortmapOpts) (io.ReadCloser, error) {
 	vals := make(url.Values)
 	if opts == nil {