Use "issue" term instead of "ticket" in DocBlocks

What is called by "issue" in JIRA is called differently in other issue trackers:
- task
- bug
- ticket
- ...

Since it's JIRA issue tracker client, then JIRA terminology should be used consistently.

I've found 3 places where `ticket` term is used instead of `issue` term in method DocBlock:
- `Api::addComment`
- `Api::getTransitions`
- `Api::transition`
