Fix Client.login documentation to showcase email/password login.
Fix Client.login documentation to showcase email/password login.

variablehair/Eggplantato,Harmon758/discord.py,imayhaveborkedit/discord.py,Harmon758/discord.py,khazhyk/discord.py,mgardne8/discord.py,lgkern/PriestPy,Rapptz/discord.py,rapptz/discord.py,Theoretical/UselessBot,jhgg/discord.py