Add case in the mount option documentation

[patrick330602]

case option is available (Ref: https://devblogs.microsoft.com/commandline/per-directory-case-sensitivity-and-wsl/) for DrvFs but it is not documented in the mount options section here(https://docs.microsoft.com/en-us/windows/wsl/wsl-config#mount-options)

[mattwojo]

Great addition. Thank you @patrick330602! Appreciate it!

[opbld33]

Docs Build status updates of commit 7761c94:

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