Clarity on mutual exclusion of Username and Token (ansible#61668)

##### SUMMARY
Although there is an example showing that username is not required with personal access token, it will be nice to more clearly state in options description. 
##### ISSUE TYPE
- Docs Pull Request

+label: docsite_pr

lib/ansible/modules/source_control/github_deploy_key.py

@@ -57,10 +57,10 @@
     default: 'no'
   username:
     description:
-      - The username to authenticate with.
+      - The username to authenticate with. Should not be set when using personal access token
   password:
     description:
-      - The password to authenticate with. A personal access token can be used here in place of a password.
+      - The password to authenticate with. Alternatively, a personal access token can be used instead of I(username) and I(password) combination.
   token:
     description:
       - The OAuth2 token or personal access token to authenticate with. Mutually exclusive with I(password).