Improve documentation of PKCS1 decryption functions

Document the preconditions on the input and output buffers for
the PKCS1 decryption functions
- mbedtls_rsa_pkcs1_decrypt,
- mbedtls_rsa_rsaes_pkcs1_v15_decrypt
- mbedtls_rsa_rsaes_oaep_decrypt

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 7e00019..a696591 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -75,6 +75,7 @@
      config-no-entropy.h to reduce the RAM footprint.
    * Added a test script that can be hooked into git that verifies commits
      before they are pushed.
+   * Improve documentation of PKCS1 decryption functions.
 
 = mbed TLS 2.5.1 released 2017-06-21