[edk2-platforms][PATCH 4/4] RPi3/RPi4: touch up readme's a bit.

["Andrei Warkentin" <andrey.warkentin@gmail.com>]

Just cleanups to areas I know have gotten a bit stale.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
---
 Platform/RaspberryPi/RPi3/Readme.md |  6 ++++--
 Platform/RaspberryPi/RPi4/Readme.md | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/Platform/RaspberryPi/RPi3/Readme.md b/Platform/RaspberryPi/RPi3/Readme.md
index b756013b..55c6265d 100644
--- a/Platform/RaspberryPi/RPi3/Readme.md
+++ b/Platform/RaspberryPi/RPi3/Readme.md
@@ -173,7 +173,7 @@ Known issues:
 ## USB
 
 - USB1 BBB mass storage devices untested (USB2 and USB3 devices are fine).
-- USB1 CBI mass storage devices don't work (e.g. HP FD-05PUB floppy).
+- Some USB1 CBI (e.g. UFI floppy) mass storage devices may not work.
 
 ## ACPI
 
@@ -187,4 +187,6 @@ install a kernel that relies on Device Tree rather than ACPI.
 ## Missing Functionality
 
 - Network booting via onboard NIC.
-- Ability to switch UART use to PL011.
+- SPCR hardcodes UART type to miniUART, and thus will not expose correct
+  (PL011) UART on CM3 and Pi2B or if DT overlays to switch UART are used
+  on Pi 3B/3B+/3A+.
diff --git a/Platform/RaspberryPi/RPi4/Readme.md b/Platform/RaspberryPi/RPi4/Readme.md
index 1f4ed001..c1ecfc99 100644
--- a/Platform/RaspberryPi/RPi4/Readme.md
+++ b/Platform/RaspberryPi/RPi4/Readme.md
@@ -121,3 +121,23 @@ An `RtcEpochSeconds` NVRAM variable is used to store the boot time.
 This should allow you to set whatever date/time you want using the Shell date and
 time commands. While in UEFI or HLOS, the time will tick forward.
 `RtcEpochSeconds` is not updated on reboots.
+
+## USB
+
+This UEFI supports both the USB3 xHCI ports (front ports), and the Pi 3-style
+DesignWare USB2 controller via the Type-C port (host only).
+
+The following only apply to the Type-C port:
+- USB1 BBB mass storage devices untested (USB2 and USB3 devices are fine).
+- Some USB1 CBI (e.g. UFI floppy) mass storage devices may not work.
+
+## ACPI
+
+OS support for ACPI description of Pi-specific devices is still in development. Not
+all functionality may be available.
+
+## Missing Functionality
+
+- Network booting via onboard NIC.
+- SPCR hardcodes type to PL011, and thus will not expose correct
+  (miniUART) UART if DT overlays to switch UART are used on Pi 4B.
\ No newline at end of file
-- 
2.17.1