doc: provide guidance on user.name format

It's a frequent misconception that the user.name variable controls authentication in some way, and as a result, beginning users frequently attempt to change it when they're having authentication troubles. Document that the convention is that this variable represents some form of a human's personal name, although that is not required. In addition, address concerns about whether Unicode is supported. Use the term "personal name" as this is likely to draw the intended contrast, be applicable across cultures which may have different naming conventions, and be easily understandable to people who do not speak English as their first language. Indicate that "some form" is conventionally used, as people may use a nickname or preferred name instead of a full legal name. Point users who may be confused about authentication to an appropriate configuration option instead. Provide a shortened form of this information in the configuration option description. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>