Remove long-obsolete text about "erase_check" affecting "flash info" output.
Move parts of that text to "protect_check", where it's still relevant; and
update the "flash info" description to mention the issue.

(This is still awkward.  It might be best to make "protect_check" mirror
"erase_check" by dumping what it finds, so "flash info" doesn't dump any
potentially-stale cache info.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>