Say on Home that the switch to 21.0.0 takes one reformatting run

The drop-in paragraph said migrating means changing the coordinates
and nothing else. Since cce474d Migrate says the fixes of 21.0.0 move
enough code, 1,023 of the 15,620 stable JDK 21 files, that the switch
is best taken with one reformatting run in a commit of its own, so
Home now says the same.

Author: abashev

docs/index.md

@@ -188,6 +188,6 @@ hook that reads the same file and runs the same check.
 ## A drop-in for palantir-java-format
 
 The output is the same as palantir-java-format 2.98.0's, except for bug fixes, and the Java packages
-are unchanged. Migrating means changing the coordinates and nothing else: [Migrate](migrate.md) lists
-every name that changes and every fix, and any later change to the output comes with a new minor
-version, see [Versions](about.md#versions).
+are unchanged. Migrating means changing the coordinates and, since 21.0.0, one reformatting run in a
+commit of its own: [Migrate](migrate.md) lists every name that changes and every fix, and any later
+change to the output comes with a new minor version, see [Versions](about.md#versions).