Fix #199: Sphinx warnings reference target not found

Fix the documentation by adding referenced-but-not-included functions and
some other small fixes.

The only warnings left are:

```
python-rsa/rsa/key.py:docstring of rsa.key.AbstractKey.load_pkcs1:: WARNING: py:class reference target not found: rsa.key.T
python-rsa/rsa/key.py:docstring of rsa.key.AbstractKey.load_pkcs1:: WARNING: py:class reference target not found: rsa.key.T
```

These are due to Sphynx not really understanding `typing` type references.
Not sure how to fix those.

Author: sybrenstuvel

doc/reference.rst

@@ -17,7 +17,13 @@ Functions
 
 .. autofunction:: rsa.find_signature_hash
 
-.. autofunction:: rsa.newkeys(keysize)
+.. autofunction:: rsa.newkeys
+
+.. autofunction:: rsa.sign_hash
+
+.. autofunction:: rsa.compute_hash
+
+.. autodata:: rsa.pkcs1.HASH_METHODS
 
 
 Classes
@@ -31,6 +37,8 @@ Classes
     constructed data. Never unpickle data received from an untrusted
     or unauthenticated source.
 
+.. autoclass:: rsa.key.AbstractKey
+
 .. autoclass:: rsa.PublicKey
     :members:
     :inherited-members:

doc/usage.rst

@@ -214,7 +214,7 @@ Modify the message, and the signature is no longer valid and a
     makes cracking the keys easier.
 
 Instead of a message you can also call :py:func:`rsa.sign` and
-:py:func:`rsa.verify` with a :py:class:`file`-like object. If the
+:py:func:`rsa.verify` with a `file`-like object. If the
 message object has a ``read(int)`` method it is assumed to be a file.
 In that case the file is hashed in 1024-byte blocks at the time.
 

rsa/pkcs1.py

@@ -57,6 +57,7 @@
     "SHA-384": hashlib.sha384,
     "SHA-512": hashlib.sha512,
 }
+"""Hash methods supported by this library."""
 
 
 if sys.version_info >= (3, 6):
@@ -423,7 +424,7 @@ def compute_hash(message: typing.Union[bytes, typing.BinaryIO], method_name: str
         object. If ``message`` has a ``read()`` method, it is assumed to be a
         file-like object.
     :param method_name: the hash method, must be a key of
-        :py:const:`HASH_METHODS`.
+        :py:const:`rsa.pkcs1.HASH_METHODS`.
 
     """