Change docs for upgrading to JIRA 7

The docs state: "Some methods on com.atlassian.jira.issue.MutableIssue appear to have been capricously renamed, at least in RC3. It’s possible that this is a mistake and it will be rectified by the final release."

Suggestion: remove "capriciously"

The name change does indeed exist in the released 7.x API and was planned. The Atlassian API was changed to use Component objects, so all methods with Objects in their name were non-GV to make that clear.