doc: provide guidance on user.name format

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>