doc: centrally document various ways tospell `true` and `false`

doc: centrally document various ways tospell `true` and `false`

We do not seem to centrally document exhaustively ways to spell
Boolean values.

The description in the Environment Variables of git(1) section
assumes that the reader is already familiar with how "Boolean valued
configuration variables" are specified, without referring to
anything, so there is no way for the readers to find out more.

The description of `bool` in the section on "--type
<type>" in "git config --help" might be the place to do so, but it
is not telling us all that much.

The description of Boolean valued placeholders in the pretty formats
section of "git log --help" enumerates the possible values with "etc."
implying there may be other synonyms; shrink the list of samples and
instead refer to the canonical and authoritative source of truth, which
now is git-config(1).

Signed-off-by: Junio C Hamano <gitster@pobox.com>