Expand documentation of `mbedtls_dhm_read_params`
diff --git a/include/mbedtls/dhm.h b/include/mbedtls/dhm.h
index 9254d95..ed39f8d 100644
--- a/include/mbedtls/dhm.h
+++ b/include/mbedtls/dhm.h
@@ -190,8 +190,13 @@
* \brief Parse the ServerKeyExchange parameters
*
* \param ctx DHM context
- * \param p &(start of input buffer), will be increased
- * by the amount of data read.
+ * \param p On input, *p must be the start of the input buffer.
+ * On output, *p is updated to point to the end of the data
+ * that has been read. On success, this is the first byte
+ * past the end of the ServerKeyExchange parameters.
+ * On error, this is the point at which an error has been
+ * detected, which is usually not useful except to debug
+ * failures.
* \param end end of buffer
*
* \return 0 if successful, or an MBEDTLS_ERR_DHM_XXX error code