Misc doc edits

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

release_notes/voltha_2.12.rst
-----------------------------
  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
diff --git a/VERSION b/VERSION
index 9d44043..965c65d 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.12.19
+2.12.20