Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/error.c b/library/error.c
index afad389..630e142 100644
--- a/library/error.c
+++ b/library/error.c
@@ -961,7 +961,7 @@
 #else /* MBEDTLS_ERROR_C */
 
 /*
- * Provide an non-function in case MBEDTLS_ERROR_C is not defined
+ * Provide a dummy implementation when MBEDTLS_ERROR_C is not defined
  */
 void mbedtls_strerror( int ret, char *buf, size_t buflen )
 {