Clarify the use of ECDSA API

In the ecdsa.c sample application we don't use hashing, we use ecdsa
directly on a buffer containing plain text. Although the text explains
that it should be the message hash it still can be confusing.

Any misunderstandings here are potentially very dangerous, because ECDSA
truncates the message hash if necessary and this can lead to trivial
signature forgeries if the API is misused and the message is passed
directly to the function without hashing.

This commit adds a hash computation step to the ecdsa.c sample
application and clarification to the doxygen documentation of the
ECDSA functions involved.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 88e2bc9..a63658b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+Changes
+   * Clarify ECDSA documentation and improve the sample code to avoid
+     misunderstandings and potentially dangerous use of the API. Pointed out
+     by Jean-Philippe Aumasson.
+
+
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
 Bugfix