Add improvements to code comments and docs

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>

include/mbedtls/x509.h

@@ -317,7 +317,7 @@ int mbedtls_x509_dn_gets(char *buf, size_t size, const mbedtls_x509_name *dn);
 
 
 /**
- * \brief            Convert the pk_key_type to a string.
+ * \brief            Return the key's type as a string.
  *
  * \param[in] pk     A mbedtls_pk_context struct containing the pk_key_type to
                      convert

library/x509.c

@@ -123,7 +123,7 @@ int mbedtls_x509_get_alg(unsigned char **p, const unsigned char *end,
 }
 
 /*
- * Convert pk_key_type to a string
+ * Convert the key type to a string
  */
 const char *mbedtls_x509_pk_type_as_string(const mbedtls_pk_context *pk)
 {