Document `deep` parameter in ExtensionArray.copy
And pass it through via `Series.copy`.

```diff
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index 108b8874b..7085a231b 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -78,6 +78,7 @@ class DecimalArray(ExtensionArray, ExtensionScalarOpsMixin):
         return self._from_sequence(result)
 
     def copy(self, deep=False):
+        print(deep)
         if deep:
             return type(self)(self._data.copy())
         return type(self)(self)
```

```python
In [1]: import pandas as pd; import numpy as np;

In [2]: from pandas.tests.extension.decimal.array import *

In [3]: s = pd.Series(DecimalArray([decimal.Decimal(1)]))

In [4]: s.copy(deep=False)
Out[4]:
0    1
dtype: decimal

In [5]: s.copy(deep=True)
False
Out[5]:
0    1
dtype: decimal

```