Merge pull request rails#50933 from 1060ki/update_docs_for_mysql_algorithm

[ci skip] [docs] Updated docs to indicate that mysql supports the algorithm

Author: fatkodima

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -876,9 +876,12 @@ def rename_column(table_name, column_name, new_column_name)
       # ====== Creating an index with a specific algorithm
       #
       #  add_index(:developers, :name, algorithm: :concurrently)
-      #  # CREATE INDEX CONCURRENTLY developers_on_name on developers (name)
+      #  # CREATE INDEX CONCURRENTLY developers_on_name on developers (name) -- PostgreSQL
       #
-      # Note: only supported by PostgreSQL.
+      #  add_index(:developers, :name, algorithm: :inplace)
+      #  # CREATE INDEX `index_developers_on_name` ON `developers` (`name`) ALGORITHM = INPLACE -- MySQL
+      #
+      # Note: only supported by PostgreSQL and MySQL.
       #
       # Concurrently adding an index is not supported in a transaction.
       #