Update dns-record-types.mdx

[smittal123]

PCX ticket : https://jira.cfdata.org/browse/PCX-20784

The current documentation (devdocs page) contains inaccurate information about DNS record name requirements.

Current (incorrect) documentation states:

FQDN must not exceed 255 characters
Must start with a letter and end with a letter or digit Only contain letters, digits, or hyphens (underscores allowed but discouraged) Issues:

255 → 253 chars: The max FQDN length in dotted notation is 253 characters (255 is the wire format including length octets)
"Start with letter" rule: This is a hostname convention (RFC 952), not a DNS protocol requirement. DNS labels can start with digits.
LDH-only restriction: DNS protocol allows any octet in labels. Underscores are common and valid (e.g., _dmarc, _acme-challenge, _domainkey).
Suggested fix: Clarify that these are recommendations for hostnames (especially if TLS certs are needed), not DNS protocol requirements. DNS itself is more permissive.

Reference thread: https://chat.google.com/room/AAAAOyUnD_g/_IwzlXZh9pY/_IwzlXZh9pY?cls=10

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