Add errata for text on plain scalar implicit keys

[eemeli]

In 7.3.3 the spec has (highlight added by me):

Plain scalars must never contain the “: ” and “ #” character combinations. Such combinations would cause ambiguity with mapping key/value pairs and comments. In addition, inside flow collections, or when used as implicit keys, plain scalars must not contain the “[”, “]”, “{”, “}” and “,” characters. These characters would cause ambiguity with flow collection structures.

But then in a BLOCK-KEY context we use the ns-plain-safe-out rule, which doesn't rule out c-flow-indicator characters.

The text is wrong here, and should be fixed to match the rules. As further evidence of this, we have test 2EBW allowing flow indicators in block map implicit keys; that's passed by all current implementations.

Fixing this disagreement by changing the rules would be a breaking change, so we can't do that.

For context, this was discovered while investigating eemeli/yaml#493, originally reported by @jedwards1211.

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