docs: note accelerator case insensitivity

[machuii]

Description of Change

Closes #38673.

Explicitly state that accelerators are case insensitive

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[jkleinsc]

Accelerators are case insensitive.
When looking up the values we convert the passed in values to lowercase to perform case insensitive lookups: eg https://github.com/electron/electron/blob/main/shell/common/keyboard_util.cc#L279

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