Documentation fix

Add relevant comments and explanations.
Fix indentation.
Add copyright notes.
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 43d001c..96d158b 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -513,7 +513,7 @@
  *                  The output is the group's OID wrapped as ASN.1.
  *
  * \param grp       ECP group used
- * \param buf       Buffer to write to
+ * \param p         Buffer to write to
  * \param size      Buffer size
  *
  * \return          Number of bytes written to \c buf,