docs: add detailed explanation about default events (#1083)

* docs: add detailed explanation about default events

* Update CHANGELOG.md

* improve redundant sentences

(cherry picked from commit 8ae897c)

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -59,3 +59,4 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Document Updates
 * (docs) [\#1059](https://github.com/Finschia/finschia-sdk/pull/1059) create ERRORS.md for x/module
+* (docs) [\#1083](https://github.com/Finschia/finschia-sdk/pull/1083) Add detailed explanation about default events

docs/docs/core/08-events.md

@@ -15,6 +15,12 @@ There are a few events that are automatically emitted for all messages, directly
 * `message.sender`: The address of the message signer.
 * `message.module`: The name of the module that emitted the message.
 
+:::tip
+`baseapp` emits exactly one `message` event for each message before any other events emitted by the message.
+The `message` event contains at least 2 attributes, exactly one `action` and exactly one `sender`.
+The position of the event may change in the next major version.
+:::
+
 :::tip
 The module name is assumed by `baseapp` to be the second element of the message route: `"cosmos.bank.v1beta1.MsgSend" -> "bank"`.
 In case a module does not follow the standard message path, (e.g. IBC), it is advised to keep emitting the module name event.