refactor(BaseService): improve prepare_request() docstring

This commit simply modifies the docstring comments related to the
`headers` and `params` arguments to the `BaseService.prepare_request()`
function in order to clarify that any entries in either of the two dictionary
arguments will simply be ignored if they have a value of None.

Signed-off-by: Phil Adams <[email protected]>

Author: padamstx

ibm_cloud_sdk_core/base_service.py

@@ -376,8 +376,10 @@ def prepare_request(
             url: The origin + pathname according to WHATWG spec.
 
         Keyword Arguments:
-            headers: Headers of the request.
-            params: Querystring data to be appended to the url.
+            headers: A dictionary containing the headers to be included in the request.
+                    Entries with a value of None will be ignored (excluded).
+            params: A dictionary containing the query parameters to be included in the request.
+                    Entries with a value of None will be ignored (excluded).
             data: The request body. Converted to json if a dict.
             files: 'files' can be a dictionary (i.e { '<part-name>': (<tuple>)}),
                 or a list of tuples [ (<part-name>, (<tuple>))... ]