Added compatibility notes on time series granularity and partial TTL indexes (#3037)

* Added compatibility notes on time series granularity and partial TTL indexes

* Typo fix and self review items

* Update source/release-notes/6.3-compatibility.txt

Co-authored-by: Fausto J Leyva <[email protected]>

---------

Co-authored-by: Fausto J Leyva <[email protected]>

Author: nvillahermosa-mdb

source/release-notes/6.3-compatibility.txt

@@ -17,3 +17,31 @@ Compatibility Changes in MongoDB 6.3
 
 This page describes changes introduced in MongoDB 6.3 that can affect
 compatibility with older versions of MongoDB.
+
+Time Series Collections
+-----------------------
+
+The following changes to :ref:`time series collections <manual-timeseries-landing>`
+can affect compatibility with older versions of MongoDB.
+
+Granularity Settings
+~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 6.3, you can configure time series bucket 
+granularity using the new ``bucketMaxSpanSeconds`` and
+``bucketRoundingSeconds`` custom bucketing parameters. To downgrade
+below 6.3, you must either drop all collections with these parameters,
+or modify them to use the corresponding ``granularity``. For details,
+see :dbcommand:`collMod`.
+
+Not all ``bucketMaxSpanSeconds`` and ``bucketRoundingSeconds`` values
+correspond to a ``granularity`` value. In such cases, you must drop the
+collection. 
+
+Partial TTL Indexes for Time Series Collections
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 6.3, you can create partial :ref:`Time To
+Live (TTL) <index-feature-ttl>` indexes on time series collections. To 
+downgrade below 6.3, you must remove all partial TTL indexes from your
+time series collections.