docs: cleanup the transaction section of README a bit (#545)

Cleans up the Transaction section of the README file a bit.

Author: olavloite

README.rst

@@ -80,7 +80,7 @@ on this step in a dialect prefix part:
    # for SQLAlchemy 1.3:
    spanner:///projects/project-id/instances/instance-id/databases/database-id
 
-   # for SQLAlchemy 1.4:
+   # for SQLAlchemy 1.4 and 2.0:
    spanner+spanner:///projects/project-id/instances/instance-id/databases/database-id
 
 To pass your custom client object directly to be be used, create engine as following:
@@ -237,7 +237,7 @@ Unique constraints
 ~~~~~~~~~~~~~~~~~~
 
 Cloud Spanner doesn't support direct UNIQUE constraints creation. In
-order to achieve column values uniqueness UNIQUE indexes should be used.
+order to achieve column values uniqueness, UNIQUE indexes should be used.
 
 Instead of direct UNIQUE constraint creation:
 
@@ -265,10 +265,16 @@ Autocommit mode
 ~~~~~~~~~~~~~~~
 
 Spanner dialect supports both ``SERIALIZABLE`` and ``AUTOCOMMIT``
-isolation levels. ``SERIALIZABLE`` is the default one, where
-transactions need to be committed manually. ``AUTOCOMMIT`` mode
-corresponds to automatically committing of a query right in its
-execution time.
+isolation levels. ``SERIALIZABLE`` is the default isolation level.
+
+``AUTOCOMMIT`` mode corresponds to automatically committing each
+insert/update/delete statement right after is has been executed.
+Queries that are executed in ``AUTOCOMMIT`` mode use a single-use
+read-only transaction. These do not take any locks and do not need
+to be committed.
+
+Workloads that only read data, should use either ``AUTOCOMMIT`` or
+a read-only transaction.
 
 Isolation level change example:
 
@@ -279,16 +285,16 @@ Isolation level change example:
    eng = create_engine("spanner:///projects/project-id/instances/instance-id/databases/database-id")
    autocommit_engine = eng.execution_options(isolation_level="AUTOCOMMIT")
 
-Automatic transactions retry
+Automatic transaction retry
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In the default ``SERIALIZABLE`` mode transactions may fail with ``Aborted`` exception. This is a transient kind of errors, which mostly happen to prevent data corruption by concurrent modifications. Though the original transaction becomes non operational, a simple retry of the queries solves the issue.
 
 This, however, may require to manually repeat a long list of operations, executed in the failed transaction. To simplify it, Spanner Connection API tracks all the operations, executed inside current transaction, and their result checksums. If the transaction failed with ``Aborted`` exception, the Connection API will automatically start a new transaction and will re-run all the tracked operations, checking if their results are the same as they were in the original transaction. In case data changed, and results differ, the transaction is dropped, as there is no way to automatically retry it.
 
 In ``AUTOCOMMIT`` mode automatic transactions retry mechanism is disabled, as every operation is committed just in time, and there is no way an ``Aborted`` exception can happen.
 
-Autoincremented IDs
-~~~~~~~~~~~~~~~~~~~
+Auto-incremented IDs
+~~~~~~~~~~~~~~~~~~~~
 
 Cloud Spanner doesn't support autoincremented IDs mechanism due to
 performance reasons (`see for more