Add special tokens to documentation for bert examples to resolve issue: #1561
**Currently the BERT examples only show the strings encoded without the inclusion of special tokens (e.g. [CLS] and [SEP]) as illustrated below:**

```
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
sentence = "Hello there, General Kenobi!"
print(tokenizer.encode(sentence))
print(tokenizer.cls_token_id, tokenizer.sep_token_id)

# [7592, 2045, 1010, 2236, 6358, 16429, 2072, 999]
# 101 102
```

**In this pull request i set ```add_special_tokens=True``` in order to include special tokens in the documented examples as illustrated below:**
```
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
sentence = "Hello there, General Kenobi!"
print(tokenizer.encode(sentence, add_special_tokens=True))
print(tokenizer.cls_token_id, tokenizer.sep_token_id)
# [101, 7592, 2045, 1010, 2236, 6358, 16429, 2072, 999, 102]
# 101 102
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=h1) Report
> Merging [#1584](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/82f6abd98aaa691ca0adfe21e85a17dc6f386497?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1584/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1584   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files          91       91           
  Lines       13593    13593           
=======================================
  Hits        11713    11713           
  Misses       1880     1880
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/1584/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `88.17% <ø> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=footer). Last update [82f6abd...1972e0e](https://codecov.io/gh/huggingface/transformers/pull/1584?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
