docs/design/: Some doc updates. Start renaming from stream_time to running_time where it was used wrongly.

Original commit message from CVS:
* docs/design/part-TODO.txt:
* docs/design/part-activation.txt:
* docs/design/part-block.txt:
* docs/design/part-buffering.txt:
* docs/design/part-clocks.txt:
* docs/design/part-element-source.txt:
* docs/design/part-events.txt:
* docs/design/part-gstbin.txt:
* docs/design/part-gstbus.txt:
* docs/design/part-gstpipeline.txt:
* docs/design/part-live-source.txt:
* docs/design/part-messages.txt:
* docs/design/part-overview.txt:
* docs/design/part-qos.txt:
* docs/design/part-query.txt:
* docs/design/part-states.txt:
* docs/design/part-trickmodes.txt:
Some doc updates. Start renaming from stream_time to running_time where
it was used wrongly.