Misc doc edits

howto/code/release-bugfix.rst
-----------------------------
  o Clarify a few items

release_notes/voltha_2.12.rst
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  o Update version and URLs for component bng.
  o Update version and URLs for component olttopology.
  o Remove pppoeagent from the VOLTHA release matrix, present in ONOS.

Change-Id: Ib035bada96466bb526674bf5bb0869a0b11a0bc5
4 files changed