Prefer 'fixed-size' to 'known-size' in doc comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 746c1d9..54819f7 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -156,7 +156,7 @@
                                size_t output_length );
 
 /**
- * \brief Conditional addition of two known-size large unsigned integers,
+ * \brief Conditional addition of two fixed-size large unsigned integers,
  *        returning the carry.
  *
  * Functionally equivalent to
@@ -189,7 +189,7 @@
                                           unsigned cond );
 
 /**
- * \brief Subtract two known-size large unsigned integers, returning the borrow.
+ * \brief Subtract two fixed-size large unsigned integers, returning the borrow.
  *
  * Calculate `A - B` where \p A and \p B have the same size.
  * This function operates modulo `2^(biL*limbs)` and returns the carry
@@ -212,7 +212,7 @@
                                        size_t limbs );
 
 /**
- * \brief Perform a known-size multiply accumulate operation: A += c * B
+ * \brief Perform a fixed-size multiply accumulate operation: A += c * B
  *
  * \param[in,out] A  The pointer to the (little-endian) array
  *                   representing the bignum to accumulate onto.