Document stdin behaviour for getting raw key

[betelgeuse]

Document the logic described in makeRawKey.

[betelgeuse]

Here's the source code logic for reference

fscrypt/cmd/fscrypt/keys.go

Lines 110 to 113 in 068b9f8

	func makeRawKey(info actions.ProtectorInfo) (*crypto.Key, error) {
	// When running non-interactively and no key was provided,
	// try to read it from stdin
	if keyFileFlag.Value == "" && !term.IsTerminal(stdinFd) {

[ebiggers]

Merged, thanks!