asn1_write documentation: say that integers must be non-negative

The documentation never said it explicitly, but the ASN.1 library
doesn't support negative integers. Say it explicitly.

Also fix a copypasta error.

Author: gilles-peskine-arm

include/mbedtls/asn1write.h

@@ -99,6 +99,7 @@ int mbedtls_asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
  * \param p         The reference to the current position pointer.
  * \param start     The start of the buffer, for bounds-checking.
  * \param X         The MPI to write.
+ *                  It must be non-negative.
  *
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
@@ -183,6 +184,7 @@ int mbedtls_asn1_write_bool( unsigned char **p, unsigned char *start,
  * \param p         The reference to the current position pointer.
  * \param start     The start of the buffer, for bounds-checking.
  * \param val       The integer value to write.
+ *                  It must be non-negative.
  *
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
@@ -231,7 +233,7 @@ int mbedtls_asn1_write_printable_string( unsigned char **p,
 
 /**
  * \brief           Write a UTF8 string in ASN.1 format using the UTF8String
- *                  string encoding tag (#MBEDTLS_ASN1_PRINTABLE_STRING).
+ *                  string encoding tag (#MBEDTLS_ASN1_UTF8_STRING).
  *
  * \note            This function works backwards in data buffer.
  *