Added changes for LEWG review

[crtrott]

• Split definition of atomic-ref-bound into one for arithmetic and pointer, and one for other types
  • needed because the typedef difference_type should only appear for arithmetic and pointer types
• Added discussion for making atomic-ref-bound exposition only or not
• Added discussion arguing for not having converting constructors with respect to memory order
• Added missing fetch_min and fetch_max functions
• Use "west const"
• Added header synopsis changes

[mhoemmen]

FYI, this version was submitted as P2689R3.