Improve documentation

Co-authored-by: Tom Cosgrove <81633263+tom-cosgrove-arm@users.noreply.github.com>
Co-authored-by: Werner Lewis <werner.wmlewis@gmail.com>
Co-authored-by: Minos Galanakis <minos.galanakis@arm.com>

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index c2c3aec..de28093 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -1,5 +1,5 @@
 /**
- *  Internal bignum functions
+ *  Modular bignum functions
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0