doc updates re Time Tracking

(1). Time Tracking now accepts fractions(see JRA-2176)
=> Update this page: http://confluence.atlassian.com/display/JIRA/Enabling+Time+Tracking
=> Mention in Release Notes
=> Don't update this page (unless fractional hours are now the system default) : http://confluence.atlassian.com/display/JIRA/Logging+Work+on+an+Issue

(2) Also see https://extranet.atlassian.com/display/JIRADEV/Time+Tracking+Improvements+for+JIRA+4.1+-+Initial+Findings

• JRA-868 - The ability to log work during an issue transition (typically Resolve)
• JRA-4888 - The ability to edit the original estimate at any time ( after having logged work even)
• JRA-1993 - The ability to set the remaining estimate to zero on transition (typically Resolve)