[pournasserian]

Closes #3073

The limit of one member mail a minute of each slot to an address is kept in IDistributedCache, so with the default MemoryDistributedCache it holds per instance. The startup report now lists member-mail-limit-per-instance (warning 9131) in Production while an email sender is set up and the resolved cache is the in-memory one, worded as a condition: it holds per instance, and with several instances a shared IDistributedCache is needed. Spec 4.7 and 4.15 (multi-instance and startup diagnostics), docs/host-configuration.md and the production setup of docs/docker.md name the requirement, the last next to the output cache.

Decisions

• Condition. forgot-password and resend-verification are open whether or not registration is, so the line does not ask AllowRegistration; it asks whether mail can go out at all (the sender is not the refusing default of Production). Without a sender no member mail goes, and the line would be noise.
• Detection. The resolved IDistributedCache is checked with is MemoryDistributedCache, the same test MemberMailLimiter makes, so the report and the limiter agree. It is resolved only in Production with a sender; the default is cheap and a Redis cache connects on first use.
• Wording, no switch. An instance cannot tell that it has peers, so the message says that one instance needs nothing. No option silences it; registering a shared cache does.
• docker.md. Its production setup did not name the output cache, so a new "Several instances" bullet names both in-memory stores and that the image keeps them per instance; running several needs a host of your own that registers shared ones.
• No new event id or code: the line is logged with the existing 9131.

Tests

• StartupReportTests.TheMemberMailLimitInTheInMemoryCacheIsReportedInProductionWhileMailGoesOut: listed in Production with SMTP and the default cache; absent without a sender, with a shared cache, and in Staging.
• Release build with -warnaserror; SDK, Core and Architecture tests; StartupReportTests; markdownlint.