Clarify the application specific usages of clockMode="local"

[plehegar]

Propose some words to allow:

1. 'whatever's local to you',
2. an arbitrary clock reference (with extra metadata to indicate the clock source).

(raised by Nigel Megitt on 2015-04-09)
From tracker issue http://www.w3.org/AudioVideo/TT/tracker/issues/383

[nigelmegitt]

Note that something similar has been done in EBU-TT Part 3 in which a reference clock identifier parameter is available for clarifying the intent behind ttp:clockMode="local".

[skynavga]

Addressed in c36dffb; closing with no further action required.

[nigelmegitt]

Noting that this was done without a pull request or review period, so members may wish to take a look as though this was "PR merged early".