create_blob_from_* functions do not exist for append blobs

### Which service(blob, file, queue) does this issue concern?
blob

### Which version of the SDK was used? Please provide the output of `pip freeze`.
2.1.0
pip freeze:
```
asn1crypto==0.24.0
azure-common==1.1.23
azure-functions==1.0.0b5
azure-storage-blob==2.1.0
azure-storage-common==2.1.0
certifi==2019.6.16
cffi==1.12.3
chardet==3.0.4
cryptography==2.7
idna==2.8
pycparser==2.19
python-dateutil==2.8.0
requests==2.22.0
six==1.12.0
urllib3==1.25.3
```

### What problem was encountered?
The documentation of `AppendBlobService` refers to `create_blob_from_*` functions in the documentation of `create_blob`. These do not exist.

This, together with #633, makes it impossible to atomically construct a new append blob with specified content.

### Have you found a mitigation/solution?
No.

Note: for table service, please post the issue here instead: https://github.com/Azure/azure-cosmosdb-python.