ovl_bg_dblue_elevator OK and documented (#1654)

* Creating branch for ovl_bg_dblue_elevator's PR.

* OK

* Screwing with my Debian install. Safety commit.

* Applied first pass of review comments to PR and added authors.

Co-authored-by: Derek-Hensley <hensley.derek58@gmail.com>
Co-authored-by: fig02 <fig02srl@gmail.com>
Co-authored-by: Kelebek1 <eeeedddccc@hotmail.co.uk>
Co-authored-by: Isghj5 <isghj5@gmail.com>
Co-authored-by: petrie911 <pmontag@PHYS-S129.iowa.uiowa.edu>
Co-authored-by: AngheloAlf <angheloalf95@gmail.com>

* Found some review comments I missed and applied the suggestions.

* Added enum for water flow and renamed waterSurfaceYPos to waterSurfacePosY

* Forgot to add last contribution.

Co-authored-by: Tom-Overton <tom.overton@outlook.com>

---------

Co-authored-by: Derek-Hensley <hensley.derek58@gmail.com>
Co-authored-by: fig02 <fig02srl@gmail.com>
Co-authored-by: Kelebek1 <eeeedddccc@hotmail.co.uk>
Co-authored-by: Isghj5 <isghj5@gmail.com>
Co-authored-by: petrie911 <pmontag@PHYS-S129.iowa.uiowa.edu>
Co-authored-by: AngheloAlf <angheloalf95@gmail.com>
Co-authored-by: Tom-Overton <tom.overton@outlook.com>

linker_scripts/undefined_syms.ld

@@ -161,11 +161,6 @@ D_0407D590 = 0x0407D590;
 
 // segment 0x06
 
-// ovl_Bg_Dblue_Elevator
-
-D_060002C8 = 0x060002C8;
-D_060005C4 = 0x060005C4;
-
 // ovl_Boss_07
 
 D_06000194 = 0x06000194;