More tweaks on EC-related wording

Use m for the bit size of the field order, not q which is
traditionally the field order.

Correct and clarify the private key representation format as has been
done for the private key and ECDH shared secret formats.
diff --git a/include/psa/crypto_sizes.h b/include/psa/crypto_sizes.h
index f4feb4d..f0a1ba7 100644
--- a/include/psa/crypto_sizes.h
+++ b/include/psa/crypto_sizes.h
@@ -422,10 +422,9 @@
  *      parameters              OBJECT IDENTIFIER } -- namedCurve
  * ECPoint ::= ...
  *    -- first 8 bits: 0x04;
- *    -- then x_P as a `ceiling(n/8)`-byte string, big endian;
- *    -- then y_P as a `ceiling(n/8)`-byte string, big endian;
- *    -- where `n` is the bit size associated with the curve,
- *    --       i.e. the bit size of `q` for a curve over `F_q`.
+ *    -- then x_P as a `ceiling(m/8)`-byte string, big endian;
+ *    -- then y_P as a `ceiling(m/8)`-byte string, big endian;
+ *    -- where `m` is the bit size associated with the curve.
  *
  * - 2 * 4 bytes of SEQUENCE overhead;
  * - 1 + 1 + 7 bytes of algorithm (id-ecPublicKey OID);