Update doc comment on acquireTokenWithClientCredentials to indicate that the new clientSecret parameter is optional.
Update doc comment on acquireTokenWithClientCredentials to indicate that the new clientSecret parameter is optional.

AzureAD/azure-activedirectory-library-for-nodejs,crwilcox/azure-activedirectory-library-for-nodejs,CedarLogic/azure-activedirectory-library-for-nodejs,CedarLogic/azure-activedirectory-library-for-nodejs,crwilcox/azure-activedirectory-library-for-nodejs,yugangw-msft/azure-activedirectory-library-for-nodejs,rjesh-git/azure-activedirectory-library-for-nodejs,AzureAD/azure-activedirectory-library-for-nodejs,AzureAD/azure-activedirectory-library-for-nodejs,yugangw-msft/azure-activedirectory-library-for-nodejs,jmspring/azure-activedirectory-library-for-nodejs,jmspring/azure-activedirectory-library-for-nodejs,rjesh-git/azure-activedirectory-library-for-nodejs