Clarify confusing use of YAML "JSON Schema"

When we mention YAML's "Failsafe schema" we give it a lower-case
"schem", as the YAML documentatio does.  We also prefix it with
"YAML".

However, we capitalize "Schema" in "JSON Schema ruleset",
which (given how much JSON Schema is used in the OAS) is a jarring
overlap with "JSON Schema".

This change aligns "YAML JSON schema ruleset" with
"YAML Failsafe ruleset" and explicitly calls out that it is
unrelated to JSON Schema.

Author: handrews

versions/3.0.4.md

@@ -148,7 +148,7 @@ Patterned fields MUST have unique names within the containing object.
 
 In order to preserve the ability to round-trip between YAML and JSON formats, YAML version [1.2](https://yaml.org/spec/1.2/spec.html) is RECOMMENDED along with some additional constraints:
 
-- Tags MUST be limited to those allowed by the [JSON Schema ruleset](https://yaml.org/spec/1.2/spec.html#id2803231).
+- Tags MUST be limited to those allowed by [YAML's JSON schema ruleset](https://yaml.org/spec/1.2/spec.html#id2803231), which defines a subset of the YAML syntax and is unrelated to [JSON Schema](https://tools.ietf.org/html/draft-wright-json-schema-00).
 - Keys used in YAML maps MUST be limited to a scalar string, as defined by the [YAML Failsafe schema ruleset](https://yaml.org/spec/1.2/spec.html#id2802346).
 
 **Note:** While APIs may be defined by OpenAPI documents in either YAML or JSON format, the API request and response bodies and other content are not required to be JSON or YAML.