mutibyte -> multibyte

document the following constants:
B_ASN1_DIRECTORYSTRING, B_ASN1_PRINTABLE, B_ASN1_DISPLAYTEXT, B_ASN1_TIME,
DIRSTRING_TYPE, PKCS9STRING_TYPE

document ASN1_tag2bit(3)

document ASN1_PRINTABLE_type(3) and ASN1_UNIVERSALSTRING_to_string(3)

document ASN1_STRING_set_by_NID(3)
and the three functions related to the global mask

new manual page ASN1_mbstring_copy(3)
also documenting ASN1_mbstring_ncopy(3)