docs: clarify closure usage in gan example (#8521)

* clarify closure usage in gan example

* Update docs/source/common/optimizers.rst

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* remove empty line

* Update docs/source/common/optimizers.rst

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

* do not capitalize if not a sentence

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

docs/source/common/optimizers.rst

@@ -396,6 +396,7 @@ For example, here step optimizer A every batch and optimizer B every 2 batches.
         # update discriminator every 2 steps
         if optimizer_idx == 1:
             if (batch_idx + 1) % 2 == 0:
+                # the closure (which includes the `training_step`) won't run if the line below isn't executed
                 optimizer.step(closure=optimizer_closure)
 
         # ...