config: document `read_early_config()` and `read_very_early_config()`

config: document `read_early_config()` and `read_very_early_config()`

It's not clear what `read_early_config()` and `read_very_early_config()`
do differently compared to `repo_read_config()` from just looking at
their names. Document both of these in the header file to clarify their
intent.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>