Fix Utf8JsonWriter WriteXValue methods documentation to remove array-specific language

[Copilot]

The WriteXValue methods in Utf8JsonWriter had documentation that incorrectly stated they write values "as an element of a JSON array". However, these methods can be used in various contexts, not just arrays:

• As array elements: [value1, value2, value3]
• As object property values: {"property": value}
• As standalone JSON values (e.g., in custom converters)

Example from the issue

The problem was evident in custom converters like this:

public override void Write(
    Utf8JsonWriter writer,
    DateTimeOffset dateTimeValue,
    JsonSerializerOptions options) =>
        writer.WriteStringValue(dateTimeValue.ToString( // Not writing to an array!
            "MM/dd/yyyy", CultureInfo.InvariantCulture));

Changes made

Updated documentation for all affected WriteXValue methods to remove the misleading "as an element of a JSON array" language:

• WriteStringValue (all overloads): DateTime, DateTimeOffset, Guid, ReadOnlySpan<byte>, ReadOnlySpan<char>, string, JsonEncodedText
• WriteBooleanValue
• WriteNullValue
• WriteNumberValue (all numeric types): Decimal, Double, Int32, Int64, Single, UInt32, UInt64
• WriteBase64StringValue

Before and after

Before:

<summary>Writes a string text value (as a JSON string) as an element of a JSON array.</summary>

After:

<summary>Writes a string text value (as a JSON string).</summary>

The documentation now accurately reflects that these methods write JSON values in any valid context, not just arrays.

Fixes #11360.

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