Impr(docs): call_end_ringtone is only triggered for inbound

[eulbat]

call_end_ringtone is only triggered for inbound

Description

Update documentation to mention call_end_ringtone is only triggered for incoming calls.

Submission

• [x] Your branch is based on master
• [ ] Your code is unit tested
• [x] CircleCI builds are green (coverage and lint)
• [x] Add team "CTI" as "Reviewer" for the PR
• [x] Name your PR with the convention fix|feat|impr(<subject>): description.

[hereismass-bot]

The code coverage of this PR is 99.21% :white_check_mark: Coverage for master is 99.21%.

[hereismass-bot]

The code coverage of this PR is 99.21% :white_check_mark: Coverage for master is 99.21%.