Clarify the meaning of "character" in the documentation

[Manlio Perillo <manlio.perillo@gmail.com>]

The term "character" is confusing: does it mean 7bit/ASCII character
or Unicode Code Point?

As an example, with
git config --add core.commentChar •  // Bullet (U+2022)
git does not complain, but it is rejected later.

A counter example is using UTF-8 with "user.name", where it is handled
correctly.

I sent this email after reading the documentation of "git diff
--color-moved=blocks, where the text says:
> Blocks of moved text of at least 20 alphanumeric characters are detected greedily.

In this case it is not clear if the number of characters are counted
as UTF-8 or normal 8bit bytes.

Thanks
Manlio Perillo