(#11535) Update user type docs w/ Windows-specific quirks

This commit adds mention of the following to the user type's doc strings:

• The windows_adsi provider manages local users, not all kinds of users.
• Windows users don't have a gid or shell attribute.
• Windows passwords are cleartext-only.
• Most platforms use sha1 for passwords, OS X Lion uses sha512.
• Puppet treats user names case-sensitively, regardless of the platform's rules.