fix(GuildMemberManager): return type can be null

[DTrombett]

Please describe the changes this PR makes and why it should be merged:

The return type of GuildMemberManager#unban was User but it just calls GuildBanManager#remove, whose return type is ?User.

This PR fixes documentation and typing for GuildMemberManager#unban to add null in its return type.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR only includes non-code changes, like changes to documentation, README, etc.