Doc: use `--type=bool` instead of `--bool`

Doc: use `--type=bool` instead of `--bool`

After fb0dc3bac1 (builtin/config.c: support `--type=<type>` as preferred
alias for `--<type>`, 2018-04-18) we have a more modern way of spelling
`--bool`.

Update all instances except those that explicitly document the
"historical options" in git-config.txt. The other old-style
type-specifiers already seem to be gone except for in that list of
historical options.

Tweak the grammar a little in config.txt while we are there.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>