docs: Add a note in Incoming email integration docs.

The note states the incoming emails are rate-limited and
its current limits.

Fixes #17435.

docs/production/email-gateway.md

@@ -29,6 +29,15 @@ polling option is convenient for testing/developing this feature
 because it doesn't require a public IP address or setting up MX
 records in DNS.
 
+```eval_rst
+   .. note::
+       Incoming emails are rate-limited, with the following limits:
+
+       * 50 emails per minute.
+       * 120 emails per 5 minutes.
+       * 600 emails per hour.
+   ```
+
 ## Local delivery setup
 
 Zulip's Puppet configuration provides everything needed to run this

zproject/computed_settings.py

@@ -382,6 +382,8 @@ RATE_LIMITING_RULES = {
 # other domains, but we use this list as an extra precaution.
 RATE_LIMITING_DOMAINS_FOR_TORNADO = ["api_by_user"]
 
+# These ratelimits are also documented publicly at
+# https://zulip.readthedocs.io/en/latest/production/email-gateway.html
 RATE_LIMITING_MIRROR_REALM_RULES = [
     (60, 50),  # 50 emails per minute
     (300, 120),  # 120 emails per 5 minutes