add :unless modifier to (doc loop)

[Andriamanitra]

:unless was added in 1.31.0 (#1289) but never documented

[sogaiu]

On a side note, there appears to be docstring content (here and elsewhere) where there are phrases such as:

• "when condition is true"
• "when condition is false"
• "if expression is falsey"
• "if expression is truthy"

In this particular docstring, there appear to be a mix (e.g. true and truthy).

Do we want to be using truthy and falsey in some of these cases where true and false are being used?

[bakpakin]

truthy/falsey is more precise here since any non-nil, non-false value will behave as true, and nil will work as false (like most conditions).

[Andriamanitra]

truthy/falsey is more precise here since any non-nil, non-false value will behave as true, and nil will work as false (like most conditions).

That's a good point – I changed both when and unless to say truthy/falsey.