OvmfPkg/PlatformBootManagerLib: rejuvenate old-style function comments

OvmfPkg/PlatformBootManagerLib: rejuvenate old-style function comments

The old-style "Routine Description: ..." comments use the leftmost column
and are placed between the parameter list and the function body. Therefore
they cause git-diff to produce bogus hunk headers that fail to name the
function being patched.

Convert these comment blocks to the current edk2 style. While at it, clean
them up too.

For PlatformBootManagerBeforeConsole() and
PlatformBootManagerAfterConsole(), copy the descriptions from the call
sites in "MdeModulePkg/Universal/BdsDxe/BdsEntry.c". They are more
detailed than the comments in the lib class header
"MdeModulePkg/Include/Library/PlatformBootManagerLib.h"; ArmVirtPkg
already uses these comments.

No functional changes.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Gabriel Somlo <gsomlo@gmail.com>