provide more guidance on symmetric keys

[fimbault]

Close #299

• soften wording on quantum protection (may)
• add " The security of this protocol when in use with symmetric keys relies on the cryptographic security of the keys themselves. Therefore, a key management system MUST be used to supply the keys. Moreover, the symmetric key MUST NOT be a human memorable password or a value derived from one."

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[fimbault]

Umm, again the editor config. For simplicity I'm just going to remove that PR and recreate it correctly.