models: Improve documentation for UserMessage.
models: Improve documentation for UserMessage.

eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,kou/zulip,kou/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,kou/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,punchagan/zulip,rht/zulip,andersk/zulip