BRANCHES.md: improve wording

Co-authored-by: Gilles Peskine <[email protected]>
Signed-off-by: Janos Follath <[email protected]>

BRANCHES.md

@@ -16,7 +16,9 @@ API compatibility in the `master` branch between major version changes. We
 also maintain ABI compatibility within LTS branches; see the next section for
 details.
 
-New LTS branches are usually created around the time of a new major version.
+Every major version will become an LTS branch when the next major version is
+released. We may occasionally create LTS branches from other releases at our
+discretion.
 After a new LTS branch is created, it remains supported for at least 3 years.
 
 ## Backwards Compatibility for application code