proper documentation for shared key requirements

doc/requirements-and-limitations.md

@@ -31,7 +31,9 @@ The `SUPER` privilege is required for `STOP SLAVE`, `START SLAVE` operations. Th
 - MySQL 5.7 `JSON` columns are not supported. They are likely to be supported shortly.
 
 - The two _before_ & _after_ tables must share some `UNIQUE KEY`. Such key would be used by `gh-ost` to iterate the table.
-  - As an example, if your table has a single `UNIQUE KEY` and no `PRIMARY KEY`, and you wish to replace it with a `PRIMARY KEY`, you will need two migrations: one to add the `PRIMARY KEY` (this migration will use the existing `UNIQUE KEY`), another to drop the now redundant `UNIQUE KEY` (this migration will use the `PRIMARY KEY`).
+  - What matters is not the key's _name_, but the columns covered by such key.
+  - As an example, if your table has a single `UNIQUE KEY my_unique_key(list, of, columns)` and no `PRIMARY KEY`, and you wish to replace the unique key with a `PRIMARY KEY`, you are good to go with a single migration: `--alter='DROP KEY my_unique_key, ADD PRIMARY KEY (list, of, columns)'`
+  - It is OK if the migrations changes type of columns within the shared key. For example, assume `id INT PRIMARY KEY`, it is OK to `--alter='MODIFY id BIGINT'`.
 
 - The chosen migration key must not include columns with `NULL` values.
   - `gh-ost` will do its best to pick a migration key with non-nullable columns. It will by default refuse a migration where the only possible `UNIQUE KEY` includes nullable-columns. You may override this refusal via `--allow-nullable-unique-key` but **you must** be sure there are no actual `NULL` values in those columns. Such `NULL` values would cause a data integrity problem and potentially a corrupted migration.