Add CHANGES entry regarding the documentation of EVP_PKEY_size() et al

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/10778)

CHANGES

@@ -9,6 +9,12 @@
 
  Changes between 1.1.1 and 3.0.0 [xx XXX xxxx]
 
+  *) Enhanced the documentation of EVP_PKEY_size(), EVP_PKEY_bits()
+     and EVP_PKEY_security_bits().  Especially EVP_PKEY_size() needed
+     a new formulation to include all the things it can be used for,
+     as well as words of caution.
+     [Richard Levitte]
+
   *) All of the low level MD2, MD4, MD5, MDC2, RIPEMD160, SHA1, SHA224, SHA256,
      SHA384, SHA512 and Whirlpool digest functions have been deprecated.
      These include: