docs: Errors documentation is missing 6 errors from table list

[titanism]

The following errors are not documented at https://github.com/nodejs/undici/blob/main/docs/api/Errors.md:

• UND_ERR
• UND_ERR_CONNECT_TIMEOUT
• UND_ERR_HEADERS_TIMEOUT
• UND_ERR_HEADERS_OVERFLOW
• UND_ERR_BODY_TIMEOUT
• UND_ERR_RESPONSE_STATUS_CODE

[ronag]

PR welcome

[ronag]

I don't know about UND_ERR? I don't think we ever throw that.

[titanism]

@ronag It is used, but only by mock agent at:

undici/lib/mock/mock-agent.js

Lines 163 to 167 in a3efc98

	throw new UndiciError(`
	${pluralizer.count} ${pluralizer.noun} ${pluralizer.is} pending:
	${pendingInterceptorsFormatter.format(pending)}
	`.trim())

@ronag We are submitting PR now, one moment!

[titanism]

@ronag see #2092