private void setValue(String value) {
        this.value = validateTextString(value);
    }
Creates a new StringFieldValue with the given value.

@param value the value to wrap.
@throws IllegalArgumentException if the string contains non-text characters as defined by 
                                 {@link Text#isTextCharacter(int)}
document/src/main/java/com/yahoo/document/datatypes/StringFieldValue.java