update the migrating doc for less confusion (#7845)

Signed-off-by: Luke Chen <showuon@gmail.com>

docs/migration.md

@@ -7,7 +7,7 @@ sort_rank: 8
 
 In line with our [stability promise](https://prometheus.io/blog/2016/07/18/prometheus-1-0-released/#fine-print),
 the Prometheus 2.0 release contains a number of backwards incompatible changes.
-This document offers guidance on migrating from Prometheus 1.8 to Prometheus 2.0.
+This document offers guidance on migrating from Prometheus 1.8 to Prometheus 2.0 and newer versions.
 
 ## Flags
 
@@ -125,7 +125,7 @@ Note that you will need to use promtool from 2.0, not 1.8.
 ## Storage
 
 The data format in Prometheus 2.0 has completely changed and is not backwards
-compatible with 1.8. To retain access to your historic monitoring data we
+compatible with 1.8 and older versions. To retain access to your historic monitoring data we
 recommend you run a non-scraping Prometheus instance running at least version
 1.8.1 in parallel with your Prometheus 2.0 instance, and have the new server
 read existing data from the old one via the remote read protocol.