sys-boot/raspberrypi-firmware: improve description

sys-boot/raspberrypi-firmware: improve description

Used the improved wording from Conrad's pull request.
The old wording might be confusing, implying that the package is only
for Raspberry Pi 1 (first generation). This confusion is not resolved
even at upstream repository's README, but apparently upstream repo
indeed supports all the generations of devices.

There are also a couple of justified spelling tweaks:
* PI -> Pi (align with official spelling),
* boot loader -> bootloader (both are in use, RPi people seem to use the
  latter).

Suggested-by: Conrad Kostecki <conrad@kostecki.com>
Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Andrey Utkin <andrey_utkin@gentoo.org>