api docs: Clean up description of legacy reaction events.
api docs: Clean up description of legacy reaction events.

* Remove `is_mirror_dummy` copy-paste error.
* Clarify explanation of the user dictionary.

kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip