Bigger EntrezAPI docstring and __init__ type hints (#31)

* Expanded docstring with better links to E-Utilities docs

* Blackened type hints, and added type hint for api_key

* Clarified tool name examples

easy_entrez/api.py

@@ -67,27 +67,42 @@ def is_response_for(response: EntrezResponse, query: Type[EntrezQueryT]) -> Type
 class EntrezAPI:
     """
     Parameters:
-        tool: Name of application making the E-utility call. Value must be a string with no internal spaces.
-        email: E-mail address of the E-utility user.
+        tool: Name of application making the E-utility call, for NCBI internal tracking.
+            Value must be a string with no internal spaces. Specify this value to
+            correspond with the name of the end-use application.
+            For example, the biopython library just uses "biopython" as the tool name, see |BioEntrez|_.
+            Please see the Frequency, Timing and Registration of E-utility URL Requests section of |EUtilsHelp|_
+            for more information on this parameter.
+        email: E-mail address of the E-utility user, for NCBI internal tracking/communications.
             Value must be a string with no internal spaces, and should be a valid e-mail address.
-        api_key: Since December 2018, NCBI began enforcing the practice of using an API key
-            for users that post more than 3 requests per second.
-            Please see `Chapter 2 <https://www.ncbi.nlm.nih.gov/books/n/helpeutils/chapter2/>`_ for a full discussion of this policy.
+            Please see the Frequency, Timing and Registration of E-utility URL Requests section of |EUtilsHelp|_
+            for more information on this parameter.
+        api_key: Since December 1st 2018, NCBI began enforcing the practice of using an
+            API key for users that post more than 3 requests per second.
+            Please see the API Keys section of |EUtilsHelp|_ for a full discussion of this policy.
         return_type: Retrieval type. Determines the format of the returned output.
         minimal_interval: The time interval (seconds) to be enforced between consecutive requests;
           by default slightly over 1/3 of a second to comply with the Entrez guidelines,
           but you may increase it if you want to be kind to others,
           or decrease it if you have an API key with an appropriate consent from Entrez.
         timeout: The timeout in seconds (default 10 seconds).
         server: The server address.
+
+    .. |EUtilsHelp| replace:: Entrez Programming Utilities Help
+    .. _EUtilsHelp: https://www.ncbi.nlm.nih.gov/books/NBK25497/
+    .. |BioEntrez| replace:: ``Bio.Entrez``
+    .. _BioEntrez: https://github.com/biopython/biopython/blob/biopython-181/Bio/Entrez/__init__.py#L141
     """
 
     def __init__(
-        self, tool: str, email: str, api_key=None,
-        return_type: ReturnType = 'json',
+        self,
+        tool: str,
+        email: str,
+        api_key: Optional[str] = None,
+        return_type: ReturnType = "json",
         minimal_interval: float = 0.334,
         timeout: float = 10,
-        server: str = 'https://eutils.ncbi.nlm.nih.gov/entrez/eutils/'
+        server: str = "https://eutils.ncbi.nlm.nih.gov/entrez/eutils/",
     ):
         self.server = server
         self.tool = tool