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>

config.c

@@ -2234,10 +2234,6 @@ void read_early_config(config_fn_t cb, void *data)
 	strbuf_release(&gitdir);
 }
 
-/*
- * Read config but only enumerate system and global settings.
- * Omit any repo-local, worktree-local, or command-line settings.
- */
 void read_very_early_config(config_fn_t cb, void *data)
 {
 	struct config_options opts = { 0 };