MdeModulePkg/UefiBootManagerLib: fix AddLoadOptionVariable docs/prototype

Clean up the leading comment and the prototype of
EfiBootManagerAddLoadOptionVariable():

- the function may modify Option on output, annotate the parameter with
  OUT and update the documentation;

- "@retval EFI_STATUS" and "@retval Others" are not idiomatic
  documentation, use @return instead;

- sync comment and prototype between lib instance and lib class header.

Change-Id: I8a609d6502b6f8929b2f568acaa147065003b6f4
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>