Add migration note on thread pool API changes

A previous change modified the output of the thread pool info contained
in the nodes info API. This commit adds a note to the migration docs for
this change.

Author: jasontedor

docs/reference/migration/migrate_7_0/api.asciidoc

@@ -22,3 +22,18 @@ The following parameters starting with underscore have been removed:
 Instead of these removed parameters, use their non camel case equivalents without
 starting underscore, e.g. use `version_type` instead of `_version_type` or `versionType`.
 
+==== Thread pool info
+
+In previous versions of Elasticsearch, the thread pool info returned in the
+<<cluster-nodes-info,nodes info API>> returned `min` and `max` values reflecting
+the configured minimum and maximum number of threads that could be in each
+thread pool. The trouble with this representation is that it does not align with
+the configuration parameters used to configure thread pools. For
+<<modules-threadpool,scaling thread pools>>, the minimum number of threads is
+configured by a parameter called `core` and the maximum number of threads is
+configured by a parameter called `max`. For <<modules-threadpool,fixed thread
+pools>>, there is only one configuration parameter along these lines and that
+parameter is called `size`, reflecting the fixed number of threads in the
+pool. This discrepancy between the API and the configuration parameters has been
+rectified. Now, the API will report `core` and `max` for scaling thread pools,
+and `size` for fixed thread pools.